implement static_mut_vars lint

[faeroon]

implements lint for checking static mut variables usage

changelog: [static_mut_vars]

• Followed lint naming conventions
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

Fixes #4070.

[github-actions]

Lintcheck changes for 0c943e8

Lint	Added	Removed	Changed
clippy::static_mut_vars	7	0	0

This comment will be updated if you push new changes

[rustbot]

r? @dswij

rustbot has assigned @dswij.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
• 7 candidates expanded to 7 candidates
• Random selection from Jarcho, dswij, llogiq, samueltardieu

[dswij]

r? clippy

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.