feat: "does not..." assertions for string

[haraldmaida]

It would be useful if the string specific assertions have a "does not..." variant.

Implemented "does not..." assertions for strings:

• does_not_contain
• does_not_start_with
• does_not_end_with
• does_not_contain_any_of
• does_not_match (requires crate feature regex)

[codecov]

Codecov Report

Attention: Patch coverage is 99.69605% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.31%. Comparing base (38c17a8) to head (96df159).

Files with missing lines	Patch %	Lines
src/string/mod.rs	99.40%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   95.71%   96.31%   +0.60%     
==========================================
  Files          31       31              
  Lines        3899     4130     +231     
  Branches     3899     4130     +231     
==========================================
+ Hits         3732     3978     +246     
+ Misses        118      112       -6     
+ Partials       49       40       -9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.