feat(lsp): add function to get semantic tokens at cursor
feat: `vim.inspect_pos()`, `vim.show_pos()` and `:Inspect[!]`

This is necessary for now to support filetypes that use a parser with a
different name (e.g. the "terraform" filetype uses the "hcl" parser).

Add a "show_tree" function to view a textual representation of the
nodes in a language tree in a window. Moving the cursor in the
window highlights the corresponding text in the source buffer, and
moving the cursor in the source buffer highlights the corresponding
nodes in the window.

"pos" has a long precedent as "position" in vim, and there is no reason
to use a verbose name here.

fix(treesitter): get_captures_at_position returns metadata

Return the full `metadata` table for the capture instead of just the
priority.

Further cleanup of related docs.

* add type annotations to code
* clean up and expand static documentation
* consistent use of tags for static and generated docs

* feat(treesitter): upstream get_{node,captures} utility functions
* refactor(treesitter): get_{nodes,captures}_at_{position,cursor}