Update tree-sitter to 0.20.8 and add arm64 platforms

[kitterion]

This is a continuation of #5 with a few changes:

• reverted an update to ruiles_nodejs. It seems a bit more involved than just changing a number, in particular it now depends on skylib. We can update rules_nodejs separately.
• node_version is now 16.9.1 which is the highest available on the 4.2.0 branch of rules_nodejs https://github.com/bazelbuild/rules_nodejs/blob/4.2.0/nodejs/private/node_versions.bzl#L1694. A version of at least 16.0.0 is needed, because that's when nodejs started including darwin-arm64 builds
• removed user_compile_flags = ["-Wno-unused-but-set-variable"]. It seems that the issue was fixed in the commit tree-sitter/tree-sitter@465ceea which is included in v0.20.8
• removed target_compatible_with = platforms. There's no need to restrict the toolchain like that, exec_compatible_with is enough.

[elliottt]

Thank you for pushing this across the finish line!