v18 packages organization

[pdubroy]

As some people may know, I've been working on a new version of Ohm based on WebAssembly. (See #511 for more details.)

I'm currently thinking about the package organization for v18. Here's a first stab at it:

The basic idea is:

• Large parts of the current "ohm-js" package get moved into a new /compiler subpackage. Generally, we expect that people will want to compile their grammars at build time (producing a .wasm + blob + a .d.ts. with types for the grammar), rather than the combined compile + load step that's used today via grammar().
• The core of the new "ohm-js" package will thus be the minimal stuff required for runtime: load a .wasm grammar, match, walk the CST.
• Other languages (Go, Python, etc.) will have a language-specific library with functionality that mirrors the "ohm-js" core.
• Eventually, there will be a rework of v17's Semantics. How this will fit in is still TBD.

Biggest open questions right now:

• Do we keep @ohm-js/cli as a separate package, or merge it into the main package? (I'm leaning towards merging.)

Any thoughts would be appreciated!