Rust syntax highlighting breaks with raw strings in attributes

[alexr00]

From @ExoticMatter in microsoft/vscode#95782

VSCode Version: 1.44.2

When raw string literals that use ##s appear in attributes in a Rust file, syntax highlighting breaks.

Steps to Reproduce:

1. Copy the following code into a Rust source file:

#[cfg(feature = r#"""#)]
const _: () = ();

You will get something similar to the following:

Does this issue occur when all extensions are disabled?: Yes

[sloane-shark]

I opened #154 to address this! But it seems to have failed in CI, and I'm not sure how to make progress getting it merged.