Fix tree sitter spans not being aligned with text after saving

[mcobzarenco]

This PR ensures that after saving a buffer, we recreate a fresh copy of the tree sitter parser.

Context: on save, we strip white-space which causes the tree sitter AST spans to not be aligned with the text any more. This is the bug reported in #42.

We are typically able to compute diffs incrementally as the buffer is edited and update the tree sitter parser accordingly. We could probably compute the diffs for the strip_trailing_whitespace function. However, in general it's expected that on save a linter or similar would be run. This makes updating the tree sitter parsers impractical.

The solution in this PR is to compute a fresh copy of the AST. This means we can run any arbitrary transformation of the text on save, without having to create a diff to update the parsers. However, the downside is that it means we need to remove the existing tree on save.

IMPORTANT: As the new tree is computed async-ly, there's (a short) period when there's no available tree. This causes some flickering on save which is not ideal.

@kevinmatthes @iainh Thank you for your investigations and patience.

Fixes #42
Based on #64

[kevinmatthes]

👍

[kevinmatthes]

By the way: testing against zee/src/main.rs to trigger the assertion shows that the flickering is not visible in any case as the time during which there is no tree is sometimes to short to even visualise it. Everything works fine!

And finally, this can be also considered a visual feedback that the saving is in progress.

@iainh, when saving large files (#58), is there a tree during the saving process? If not so, we could declare this a feature to warn the users that their files might be lost when quitting without syntax highlighting.