Add scripts and instructions for switching between tree-sitter versions

[mjambon]

Motivation

This will facilitate regenerating grammar code using the old tree-sitter (0.20.6) as required by many target languages in ocaml-tree-sitter-semgrep. I need to regenerate them because I'm changing the format of the parse results to accommodate the so-called extras.

Changes

• The default tree-sitter version to use is now kept in the file tree-sitter-version.default instead of being in a script.
• The actual tree-sitter version being used is in the file tree-sitter-version.
• We provide a script ./scripts/switch-tree-sitter-version for developers of ocaml-tree-sitter-semgrep who need to switch back and forth between tree-sitter versions (without having to recompile everything).

Compatibility

Building and installing the tree-sitter and ocaml-tree-sitter runtime libraries - as needed by semgrep - is unchanged. The default destination folder of the tree-sitter binaries and libraries is still tree-sitter (except that it's now a symlink).

Tests

Test for the default tree-sitter version (0.22.6):

make setup && make && make install && make test
cat tree-sitter-version  # should be 0.22.6

Test for the older tree-sitter version (0.20.6):

./scripts/switch-tree-sitter-version 0.20.6
cat tree-sitter-version  # should be 0.20.6
make setup && make && make install && make test

Check that re-running make setup is faster the second time around for a given tree-sitter version.

Documentation

This situation is temporary and hopefully won't happen again. This is due to the recent tree-sitter no longer supporting C++ scanners. I added a minimal paragraph about version management to the readme.

Security

• Change has no security implications (otherwise, ping the security team)