A library of common pattern restrictions

Pattern restrictions are incredibly fiddly. It would be nice to have a library of ready-made patterns for common things.

I'll just start dumping patterns here:

Complex number ins real-imaginary form `a/c+(b/c)i` or `(a+bi)/c`, where a, b and c are integers:
```
[int: `+-integer:$n] `@ (
(`+-((int/int`?))`? + `+-((int`?/int`?)*i)`?)
`|
(`+-(int)`? + `+-(int`? * i)`?)/int`?
)
```

No files or tests are named. Start by locating the editor's existing pattern-restriction handling and review the complex-number example in issue #570. Confirm the scope and supported common patterns with maintainers; done means a defined, usable library of ready-made restrictions.

Written by the indexing model from the issue text.