This matches the `iter_captures` functionality to the `iter_matches`
functionality, allowing it to specify a match limit and start depth for
the query iterator.

Problem: Cannot disable individual captures and patterns in treesitter queries.

Solution: 
* Expose the corresponding tree-sitter API functions for `TSQuery` object. 
* Add documentation for `TSQuery`.
* Return the pattern ID from `get_captures_at_pos()` (and hence `:Inspect!`).

**Problem:** Some weird languages have different comment syntax
depending on the location in the code, and we do not have a way to
determine the correct `commentstring` for these special cases.

**Solution:** Allow queries to specify `commentstring` values in
metadata, allowing users/`nvim-treesitter` to provide a better
commenting experience without hugely increasing the scope of the code in
core.

TSHighlighter now places marks for conceal_lines metadata. A new
internal decor provider callback _on_conceal_line was added that
instructs the highlighter to place conceal_lines marks whenever the
editor needs to know whether a line is concealed. The bundled markdown
queries use conceal_lines metadata to conceal code block fence lines.

Co-authored-by: Jaehwang Jung <tomtomjhj@gmail.com>

Problem:
`yx` uses "y" prefix, which shadows a builtin operator.

Solution:
Use `g=` (in the form of `g==` currently), drawing from precedent of
CTRL-= and 'tpope/vim-scriptease'.