feat(lsp)!: vim.lsp.inlay_hint.get(), enable(), is_enabled() #25512

refactor!: `vim.lsp.inlay_hint()` -> `vim.lsp.inlay_hint.enable()`

Problem:
The LSP specification allows inlay hints to include tooltips, clickable
label parts, and code actions; but Neovim provides no API to query for
these.

Solution:
Add minimal viable extension point from which plugins can query for
inlay hints in a range, in order to build functionality on top of.

Possible Next Steps
---

- Add `virt_text_idx` field to `vim.fn.getmousepos()` return value, for
  usage in mappings of `<LeftMouse>`, `<C-LeftMouse>`, etc

scripts/gen_vimdoc.py

@@ -259,6 +259,7 @@ CONFIG = {
             'buf.lua',
             'diagnostic.lua',
             'codelens.lua',
+            'inlay_hint.lua',
             'tagfunc.lua',
             'semantic_tokens.lua',
             'handlers.lua',