**Problem:** The treesitter `foldexpr` calls `get_parser()` for each
line in the buffer when calculating folds. This can be incredibly slow
for buffers where a parser cannot be found (because the result is not
cached), and exponentially more so when the user has many
`runtimepath`s.
**Solution:** Only fetch the parser when it is needed; that is, only
when initializing fold data for a buffer.
Co-authored-by: Jongwook Choi <wookayin@gmail.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>
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