Add inlay hint support per LSP specification.

[xeger]

Closes #103.

I'm retaining the support for the old-style RPC for now to avoid breaking the VS Code extension. I guess we'd technically increment the minor version of syntax_tree, release a new cut of the extension, and then increment the major version of syntax_tree when removing support for the proprietary stuff! What a pain.

It begs the question: should the LSP server be built into the syntax_tree gem, if it's likely to change frequently? I suppose that depends on the future of the parsing and formatting functions -- if some or all of the gem will be accepted as a built-in gem, then perhaps that would be a good time to split it up.

[kddnewton]

I'm not confident that core will ever want to actually merge this tbh, so I doubt it'll become a default gem unfortunately.

Yeah, let's go through the necessary process that you described above. I'll release a minor version after I merge this.