Implement textDocument/formatting

[cannorin]

This PR implements textDocument/formatting with a wrapper module to ocamlformat.

It finds ocamlformat from the current PATH and runs it at the current directory ocamllsp is running. Note that I didn't add any facility to find the nearest .ocamlformat.

textDocument/{rangeFormatting, onTypeFormatting} is not yet implemented. Once ocaml-ppx/ocamlformat#1188 comes into reality, we can easily extend this implementation to support them.

[rgrinberg]

Looks good, but let's get rid of all the things used to configure ocamlformat. Ocamlformat should read its own configuration by reading the .ocamlformat file (or otherwise choose a default) config.

[cannorin]

OK, I'll remove those config stuffs.

[cannorin]

We will have to set --lines option to support textDocument/{rangeFormatting, onTypeFormatting}, so I'll leave minimal support of passing options instead of completely removing it.

[rgrinberg]

Looking good. In this PR, we still need to add:

• Tests (we may install ocamlformat for this)
• Change the capabilities of the server to signal to the client that it can format documents.

[cannorin]

Ok, but where the convention is documented?

[rgrinberg]

Ok, but where the convention is documented?

Nowhere 😭 Once I switch to generated code, I will document the convention more thoroughly. For now, the simple rule is: any identifier from the LSP spec should maintain its casing. Otherwise, we should stick to the OCaml convention of [Ff]oo_bar

[cannorin]

rebased.

[cannorin]

@rgrinberg I revert efe2672. Stdune's read_all uses in_channel_length which is only meaningful for channels connected to regular files. In our case the channel is connected to the stdout of ocamlformat and it raises Sys_error("Illegal seek").

[cannorin]

I guess we should add ocamlformat to the CI environment and I'm not sure how.

[rgrinberg]

I pushed a commit that handles errors a little better notifies the user if the ocamlformat binary is missing.