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>

Problem: When there is a tree view opened by :InspectTree and the
source buffer is open in multiple windows, closing one of the source
windows will lead to the tree view being closed as well.
Regression by #31181.

Solution: Check how many source windows are open when trying to
quit one. If there are more than one, keep the tree view(s) open.
If the only source window is closed, also close the tree view(s).

fix #31196

Problem: Quitting source buffer for ```:InspectTree``` command raises
```E855``` when source buffer and tree views are the only open buffers.

Solution: Add ```QuitPre``` event to autocmd handling closing/hiding the
source buffer to close all open tree views. This allows nvim to quit
when source and tree buffers are the only open windows.

Problem:  The window opened :InspectTree or :EditQuery isn't closed when
          the source buffer is unloaded, even though it is closed when
          the buffer is hidden.
Solution: Also close the window on BufUnload.

- Also fix `vim.validate()` for PUC Lua when showing errors for values
  that aren't string or number.

Problem: For :InspectTree, indent size (`&shiftwidth`) for the tree
viewer may be incorrect.

This is because the tree viewer buffer with the filetype `query` does
not explicitly configures the tab size, which can mismatch with the
default indent size (2) assumed by TSTreeView's implementation.

Solution: Set shiftwidth to be the same as TSTreeViewOpts specifies,
which defaults to 2.