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.

未指定源文件、测试或入口点。首先将 LSIF 和 SCIP 规范与 LuaLS 已经提供的信息进行比较，然后确定生成项目索引的范围。当 LuaLS 能够将项目信息提取到受支持的索引格式中，并且该行为由测试覆盖时，即视为完成。