to ease debug. At one point I had an empty filetype and the current message was not helpful enough

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

This is essentially a convenience wrapper around the `pending()`
function, similar to `skip_fragile()` but more general-purpose.

Also remove `pending_win32` function as it can be replaced by
`skip(iswin())`.

"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.

The @error capture is used for tree-sitter's ERROR node, which indicates
a parsing error -- which can be quite frequent (and jarring) while typing.

Users can still manually `hi link @error Error` in their config.