Add markupsafe.Markup XSS plugin

[Daverball]

Closes #1067

This adds a plugin matching the existing Ruff lint RUF035, including its two settings.

[Daverball]

On an unrelated note: I've noticed that a lot of the call-related methods on Context (especially the ones for inspecting arguments) are completely broken. They're not used anywhere and their unit tests are heavily mocked. I'd suggest getting rid of them, since they only serve as pitfalls right now. If you want to keep them I would encourage you to switch to more representative tests using ast.parse.