feature: add assert_matches as unstable feature

[tommilligan]

Closes #80

Inlines the existing top level implementation of stdlib's assert_matches, and links it into our existing display framework. The results look... okay. Patterns don't come with nice multiline displaying, so we're mostly diffing a value against a pattern - but just getting them displayed in red/green and on separate lines is still nice.

@gilescope this was your suggestion, so will leave open for a bit if you have any further thoughts?

[codecov-commenter]

Codecov Report

Merging #81 (cf21269) into main (55f9b7a) will not change coverage.
The diff coverage is n/a.

❗ Current head cf21269 differs from pull request most recent head d084599. Consider uploading reports for the commit d084599 to get more accurate results

@@           Coverage Diff           @@
##             main      #81   +/-   ##
=======================================
  Coverage   96.95%   96.95%           
=======================================
  Files           4        4           
  Lines         197      197           
=======================================
  Hits          191      191           
  Misses          6        6           

Impacted Files	Coverage Δ
pretty_assertions/src/lib.rs	100.00% <ø> (ø)
pretty_assertions/tests/macros.rs	98.18% <ø> (ø)

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[gilescope]

That's smashing! You've made my day. Yes closes the issue.