**Problem:** In the case where the user sets the treesitter foldexpr upon startup in
their `init.lua`, the fold info will be calculated before the parser has
been loaded in, meaning folds will be properly calculated until edits or
`:e`.

**Solution:** Refresh fold information upon `VimEnter` as a sanity check
to ensure that a parser really doesn't exist before always returning
`'0'` in the foldexpr.

**Problem:** The treesitter `foldexpr` runs synchronous parses to
calculate fold levels, which eliminates async parsing performance in the
highlighter.

**Solution:** Migrate the `foldexpr` to also calculate and apply fold
levels asynchronously.

**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>

Problem: autocmd to refresh folds always uses the current buffer if the
option type is local. However, the current buffer may not have a parser,
and thus the assert that checks for a parser could fail.

Solution: check if the foldinfo contains the buffer, and only refresh if
so.