perf(treesitter): cache fold query (#24222)

perf(treesitter): cache vim.treesitter.query.get

Problem:
vim.treesitter.query.get searches and reads query files every time it's
called, if user hasn't overridden the query. So this can incur slowdown
when called frequently.

This can happen when using treesitter foldexpr. For example, when using
`:h :range!` in markdown file to format fenced codeblock, on_changedtree
in _fold.lua is triggered many times despite that the tree doesn't have
syntactic changes (might be a bug in LanguageTree). (Incidentally, the
resulting fold is incorrect due to a bug in `:h range!`.) on_changedtree
calls vim.treesitter.query.get for each tree changes. In addition, it
may request folds queries for injected languages without fold queries,
such as markdown_inline.

Solution:
* Cache the result of vim.treesitter.query.get.
* If query file was not found, fail quickly at later calls.

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