Now `:InspectTree` will show missing nodes as e.g. `(MISSING identifier)`
or `(MISSING ";")` rather than just `(identifier)` or `";"`. This is
doable because the `MISSING` keyword is now valid query syntax.

Co-authored-by: Christian Clason <c.clason@uni-graz.at>

When given, only that range will be checked for validity rather than the
entire tree. This is used in the highlighter to save CPU cycles since we
only need to parse a certain region at a time anyway.

This means that all work previously done by a `_parse()` iteration will
be kept in future iterations. This prevents it from running indefinitely
in some cases where the file is very large and there are 2+ injections.

Problem:
When running an initial parse, parse() returns an empty table rather
than an actual range. In `languagetree.lua`, we manually check if
a parse was incremental to determine the changed parse region.

Solution:
- Always return a range (in the C side) from parse().
- Simplify the language tree code a bit.
- Logger no longer shows empty ranges on the initial parse.

This simplifies some logic in `languagetree.lua`, removing the need for
`_has_regions`, and removing side effects in `:included_regions()`.

Before:
- Edit is made which sets `_regions = nil`
- Upon the next call to `included_regions()` (usually right after we
  marked `_regions` as `nil` due to an `_iter_regions()` call), if
  `_regions` is nil, we repopulate the table (as long as the tree
  actually has regions)

After:
- Edit is made which resets `_regions` if it exists
- `included_regions()` no longer needs to perform this logic itself, and
  also no longer needs to read a `_has_regions` variable

Co-authored-by: Dustin S. <dstackmasta27@gmail.com>
Co-authored-by: Ferenc Fejes <fejes@inf.elte.hu>
Co-authored-by: Maria José Solano <majosolano99@gmail.com>
Co-authored-by: Yochem van Rosmalen <git@yochem.nl>
Co-authored-by: brianhuster <phambinhanctb2004@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem: Error occurs if delete buffer in the middle of parsing.
Solution: Check if buffer is valid in parsing.

Problem:  `TSHighlighter:destroy()` causes double-processing of the modeline
and failure of `b:undo_ftplugin`.

Solution: Disable modeline in `TSHighlighter:destroy()` by setting `modeline=false`
if executing `syntaxset` autocommands for the `FileType` event.

Co-authored-by: Daniel Petrovic <daniel.petrovic@ebcont.com>