Support generating SCIP/LSIF index

The Language Server Index Format and its (kind of) successor SCIP define language-independent formats for information dumps of a language server. It allows tools like code editors and online repositories (eg. supported by GitHub, GitLab) to provide some LSP features without requiring an entire language server instance running in the background.

The LSIF specification has currently not reached 1.0.0 status yet.
In its most recent specification, LSIF would allow for the following language features in offline situations:

• textDocument/definition
• textDocument/declaration
• textDocument/hover
• textDocument/references
• textDocument/implementation
• textDocument/typeDefinition
• textDocument/foldingRange
• textDocument/documentLink
• textDocument/documentSymbol
• textDocument/semanticTokens/full

I'm not entirely sure whether this topic has been discussed in this community before, I wasn't able to find any mentions here in the older issues. In my opinion it could be a great feature to have a way of extracting project information out of the LuaLS into LSIF files.

ソースファイル、テスト、エントリポイントのいずれも指定されていません。まず LSIF および SCIP の仕様を、LuaLS がすでに公開している情報と比較し、次にプロジェクトインデックスの生成範囲を決定してください。LuaLS がプロジェクト情報をサポートされているインデックス形式に抽出でき、その動作がテストでカバーされていれば完了です。