vimscript introspection / meta structure

[justinmk]

Problem

Where should "core introspection" stuff live? Currently we have introspection interfaces in these places:

1. context.c
2. nvim_get_api,
3. nvim_get_commands, nvim_get_functions
4. src/nvim/eval.lua (internal)

Proposal

1. ✅ context.c (nvim_get_context) is for mutable state that can be saved and restored.
   • variables, regs, jumps, bufs, gvars, …
2. ✅ nvim_get_api is for C/RPC API only.
3. ✅ user-defined Vimscript functions/commands are introspectable via:
   • nvim_get_commands
   • nvim_get_functions
4. runtime/…/_meta/*.lua provides docstring annotations for internal functions.
   • ✅ _meta/options.lua provides annotations for options. PR #24528
   • ✅ _meta/vimfn.lua provides annotations for builtin Vimscript functions. PR #24493
   • TODO: _meta/vimcmd.lua provides annotations for builtin Ex commands. #26903 PR #27111
   • TODO: _meta/vimnormal.lua provides annotations for builtin Normal commands. #8029 #18750

TODO

• Document the above.
• Document runtime/doc/api.mpack and related runtime/doc/*.mpack files that we already ship!
• Provide a util function that makes it easy for plugins to get a Lua structure from docstring annotations.
  • This allows "which-key" and fuzzy-finder plugins to easily present the info from _meta/vimfn.lua.