feat: .test() boolean shortcut on Pattern and RegexBuilder (#264)

Adds the last convenience terminal from #127: `.test(string) -> bool` on
both `Pattern` and `RegexBuilder`.

## What is new
- `test(string, pos=0, endpos=sys.maxsize) -> bool` on `MatcherMixin` —
returns `True` when the pattern matches anywhere in the input, else
`False`. Semantically `self.to_regex().search(...) is not None`.

## Why
PR #263 already delivered `.match`, `.search`, `.fullmatch`, `.findall`,
`.finditer`, `.sub`, `.subn`, `.split` from the #127 wishlist. `.test` —
a boolean shortcut idiomatic in Ruby/humre/JS — was the only piece
missing.

## Example
```python
from edify import DIGIT, START, END, exactly

zip_code = START + exactly(5, DIGIT) + END
zip_code.test("10001")           # True
zip_code.test("nope")            # False
```

Closes #127

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