Add polyfill for Regex.Count(...)

[Tyrrrz]

No description provided.

[Copilot]

Pull request overview

This PR adds polyfill implementations for the Regex.Count(...) methods introduced in .NET 7.0, enabling their use in earlier .NET versions.

Key Changes:

• Implements four Regex.Count(...) overloads: one instance method and three static methods with varying parameters
• Adds comprehensive test coverage for all overloads
• Updates documentation with the new API signatures

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
PolyShim/Signatures.md	Updates totals to reflect 4 new member signatures and documents the new Regex.Count overloads
PolyShim/Net70/Regex.cs	Implements polyfills for instance and static Count methods using Match/NextMatch loop and Regex.Matches(...).Count
PolyShim.Tests/Net70/RegexTests.cs	Adds test coverage for all Count overloads including instance, static, with options, and with timeout variants

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (23ded1f) to head (4494c82).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@     Coverage Diff      @@
##   master   #54   +/-   ##
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