Fix a crash when `# fmt: on` is used on a different block level than `# fmt: off` (#3281)

Previously _Black_ produces invalid code because the `# fmt: on` is used on a different block level.

While _Black_ requires `# fmt: off` and `# fmt: on` to be used at the same block level, incorrect usage shouldn't cause crashes.

The formatting behavior this PR introduces is, the code below the initial `# fmt: off` block level will be turned off for formatting, when `# fmt: on` is used on a different level or there is no `# fmt: on`. This also matches the current behavior when `# fmt: off` is used at the top-level without a matching `# fmt: on`, it turns off formatting for everything below `# fmt: off`.

- Fixes #2567
- Fixes #3184
- Fixes #2985
- Fixes #2882
- Fixes #2232
- Fixes #2140
- Fixes #1817
- Fixes #569