fix: parse doc strings with litrs

[adalinesimonian]

Fixes #885

[lilizoey]

is there a way you could add a test for this case to verify that it's fixed?

[adalinesimonian]

register_docs_test.rs checks the formatting and would catch this case, the test just needs to be updated since right now it would fail expecting a ´\"´. I'm changing the test in PR #1017 so there may be a conflict between these two PRs, I can update the test here and then resolve the conflict in the other branch after this one is merged given that this one is small and easier to review, or I could do it the other way and leave this branch until the other one is merged and then rebase onto main and patch the fix. Any preference one way or another?

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1015