The submodules system is being replaced with a command-line flag

    hx --fetch-grammars

Which shallow-clones grammar repositories at the given revision and

    hx --build-grammars

For building grammars separate of the initial compilation of helix.

Why remove submodules?

* Cloning helix in general takes a long time because of the submodules,
  especially when the submodules are not fetched as shallow
* Packaging is consistently painful no matter the package-manager
* It is quite difficult to devise a scheme where users can declare
  a desired set of grammars and implement it with submodules

This commit fully removes the existing tree-sitter submodules from
the tree (as well as the .gitmodules file which is no longer used).

Here we add syntax to the languages.toml languge

    [[grammar]]
    name = "<name>"
    source = { .. }

Which can be used to specify a tree-sitter grammar separately of
the language that defines it, and we make this distinction for
two reasons:

* In later commits, we will separate this code from helix-core
  and bring it to a new helix-loader crate. Using separate schemas
  for language and grammar configurations allows for a nice divide
  between the types needed to be declared in helix-loader and in
  helix-core/syntax

* Two different languages may use the same grammar. This is currently
  the case with llvm-mir-yaml and yaml. We could accomplish a config
  that works for this with just `[[languages]]`, but it gets a bit
  dicey with languages depending on one another. If you enable
  llvm-mir-yaml and disable yaml, does helix still need to fetch and
  build tree-sitter-yaml? It could be a matter of interpretation.

helix-syntax mostly existed for the sake of the build task which
checks and compiles the submodules. Since we won't be relying on
that process anymore, it doesn't end up making much sense to have
a very thin crate just for some functions that we could port to
helix-core.

The remaining build-related code is moved to helix-term which will
be able to provide grammar builds through the --build-grammars CLI
flag.

This is not strictly speaking necessary. tree_sitter_library was used by
just one grammar: llvm-mir-yaml, which uses the yaml grammar. This will
make the language more consistent, though. Each language can explicitly
say that they use Some(grammar), defaulting when None to the grammar that
has a grammar_id matching the language's language_id.

build_grammars adapts the functionality that previously came from
helix-syntax to be used at runtime from the command line flags.

fetch_grammars wraps command-line git to perform the same actions
previously done in the scripts in helix-editor#1560.

The vision with 'use-grammars' is to allow the long-requested feature
of being able to declare your own set of grammars that you would like.
A simple schema with only/except grammar names controls the list
of grammars that is fetched and built. It does not (yet) control which
grammars may be loaded at runtime if they already exist.

This is a rather large refactor that moves most of the code for
loading, fetching, and building grammars into a new helix-loader
module. This works well with the [[grammars]] syntax for
languages.toml defined earlier: we only have to depend on the types
for GrammarConfiguration in helix-loader and can leave all the
[[language]] entries for helix-core.

This is a bit of a micro-optimization: in the current setup we waste
a thread in the pool for a local grammar only to println! a message
saying we're skipping fetching because it's a local grammar.

This restores much of the behavior that existed before this PR:
helix will build the grammars when compiling. The difference is that
now fetching is also done during the build phase and is done much
more quickly - both shallow and in parallel.

This commit replaces the out-of-date builder in the flake which relied
on submodules for fetching and the compiler for building. Now we
disable fetching and building explicitly with the environment variable
and then use builtins.fetchGit and a derivation mostly derived from
upstream to compile the grammars.

Anecdotally, this is still quite slow as builtins.fetchGit does not
seem to do shallow clones. I'm not sure I see a way around it though
without recording sha256s, which seems cumbersome.

The old flags were a bit long. --grammar is also aliased to -g to make
it even easier.

Here we perform a shallow fetch using builtins.fetchTree. In order
to make this work, we need to specify the `ref' for any repository
that doesn't have `master' as its default branch (I'm not sure why
this limitation exists since we don't need this when performing
the shallow fetch in `--grammar build')

This `ref' field is ignored by helix, so I have left it undocumented
for now, but I could be open to documenting it.

Looks like this was rebased a few hours ago and now the 789a171
revision no longer exists.