feat(treesitter): use weak tables when possible

Also add the defaulttable function to create a table whose values are created when a key is missing.

Doing so on `BufDelete` has issues:
  - `BufDelete` is also fired for listed buffers that are made unlisted.
  - `BufDelete` is not fired for unlisted buffers that are deleted.

This means that diagnostics will be lost for a buffer that becomes unlisted.

It also means that if an entry exists for an unlisted buffer, deleting that
buffer later will not remove its entry from the cache (and you may see "Invalid
buffer id" errors when using diagnostic functions if it was wiped).

Instead, remove a buffer from the cache if it is wiped out.
This means simply `:bd`ing a buffer will not clear its diagnostics now.

Co-authored-by: Famiu Haque <famiuhaque@protonmail.com>

Co-authored-by: Gregory Anders <greg@gpanders.com>

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

This makes callback_call() match call_vim_function() when calling a function.

These were just added to avoid churn when changing the default
of 'display'. To simplify message handling logic, we might want
to remove support for printing messages in default_grid later on.
This would allow things like printing error messages safely in the
middle of redraw, or a future graduation of the 'multigrid' feature.