Support textDocument/codeLens request in LSP

[goodmind]

Proposal

Support textDocument/codeLens request

• textDocument/codeLens
• codeLens/resolve

Use case

Support type annotations for top-level variables in codeLens like ReasonML and OCaml language servers

[goodmind]

• Worst case
  
  

• Normal case
  

[vicapow]

For context for others, is was discussed that the output in most cases might be too verbose. We could consider adding this behind some sort of configuration Flag

[mroch]

we should talk through what specific code lenses we'll support. agree that the signature might be too verbose, or redundant. but there are other lenses like find-refs that seem like a good fit.

[mostafaeweda]

Using CodeLens for type inference or hints might be noisy or too verbose.

While that way might be useful for languages where annotations are optional (OCaml) to demonstrate value, when possible; in other languages with strong-typing (Java, C++) or encouraged-typing, like Flow here (or TypeScript's use of CodeLens in references), it'd be better to avoid the noise and save the editor-space for code.

• Java: https://github.com/redhat-developer/vscode-java/blob/master/package.json#L166
• cquery: https://github.com/cquery-project/vscode-cquery/blob/master/package.json#L901
• TypeScript: https://github.com/microsoft/vscode/blob/master/extensions/typescript-language-features/package.json#L102