Change Rust grammar source

[sebastinez]

Description

Replace the zargony/atom-language-rust grammar submodule with dustypomerleau/rust-syntax, due to the former not being actively maintained.
This issue has been already talked about in this discussion as a good choice #5965 (comment).

This changeset has been created by the script/add-grammar script.

Since the upstream highlighting engine now implements the treesitter Rust grammar, I understand that this PR is not that high priority, but there are tools and packages downstream that take linguist as the their source.

Checklist:

• I am changing the source of a syntax highlighting grammar
  • Old: https://github.com/zargony/atom-language-rust
  • New: https://github.com/dustypomerleau/rust-syntax.git

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.

[sebastinez]

Thanks @lildude , much appreciated