fuzz: add harness for filters and escape

[manunio]

This pr:

• Adds a harness for filters and escape, this has helped in finding bugs like https://github.com/djc/askama/pull/1067
• Moves all fuzz targets to a root fuzz crate, I think this approach is better than creating it per crate.

[manunio]

Fuzzing failure will be solved once this pr and the google/oss-fuzz#12084 merges.
Edit: All checks are green at oss-fuzz so this is safe to merge.

[djc]

Would you mind making this two separate commits (or PRs)? One that moves the existing parser fuzz tests to the new crate, and a separate one that adds the new filter tests.

[manunio]

Would you mind making this two separate commits (or PRs)? One that moves the existing parser fuzz tests to the new crate, and a separate one that adds the new filter tests.

Sure, no problem.