Move completions rendering into a separate module

[popzxc]

This PR extracts rendering-related things from Completions structure to the new render module.

render module declares a Render structure (which is a generic renderer interface), RenderContext (interface for data/methods not required for completions generating, but required for rendering), and a bunch of smaller *Render structures which encapsulate logic behind rendering a certain item.

This is just a step in full separation direction, since the following this are still to be done:

• Move some data from CompletionContext to the RenderContext;
• Forbid any kind of rendering outside of render module;
• Extract score computing into a separate module.

This PR is already pretty big, so not to make it even harder to review I decided to split this process into several subsequent PRs.

[matklad]

Looks good to me, this is definitely a direction worth exploring. I like the stricter separation, and I like that each thing gets its dedicated module.

I slightly don't like the proliferation of types here. There's a natural tension between "every bit of logic is separated into a module" and "there's only few vocabulary types". In an ideal world, there should be many pieces, but they all should operate the same types.

I am also not sure if super-strict separation would be benefitial long-term (although i expect that it would).

The only not-so-minor complaint I have is about do-er objects, the rest looks good to me!

bors d+

[bors]

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[popzxc]

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)