PR Summary

switch -regex no longer rebuilds a Regex object for every line that matches, making
regex switch statements substantially faster and much lighter on allocation.

SwitchOps.ConditionSatisfiedRegex paired a static Regex.Match call with new Regex(...)
so it could read the group names, on the assumption — stated in the code comment — that the
constructor would hit .NET's regex cache. Only the static Regex methods consult that cache,
so every successful match paid a full pattern parse and matcher codegen. The guard did not
help either, since m.Groups.Count is at least 1 for any successful match.

This routes the branch through ParserOps.NewRegex, the cache that -match, -replace and
-split already use, and matches on that instance. The pattern and RegexOptions are
unchanged, so matching semantics are identical, and the existing catch (ArgumentException)
still fires for invalid patterns.

Also adds test/powershell/Language/Scripting/SwitchRegex.Tests.ps1. switch -regex had no
test coverage anywhere under test/powershell.

Fix #27975

PR Context

Reported in #27975. Measured on a local Release build of 6ca24ccf0, 200,000-line log,
median of 5 runs, both binaries built the same way:

The second row is a separate effect: because the old code went through the static cache,
which holds only Regex.CacheSize (15) patterns, a switch with more distinct clause patterns
than that recompiled every clause for every line. Allocation is the most stable signal — it is
identical at 1008.6 MB on both the unpatched local build and the shipped 7.6.5 release.

One design point worth a reviewer's attention, also raised on the issue: clause patterns are
now retained in a process-wide cache of up to 1000 entries rather than the static cache's 15,
and [regex]::CacheSize no longer influences switch -regex. That makes switch consistent
with the other regex operators, but it is a policy change rather than a pure optimization, so
I would rather flag it than have it found in review.

Verification: the new test file covers named and numbered groups, the exact $matches key
set, $matches left untouched on a non-match, default case-insensitivity, -CaseSensitive, a
[regex] instance as the clause condition under both switch modes, clause fallthrough, the
InvalidRegularExpression error id, switch -regex -file, and 24 distinct clause patterns
over repeated passes. It passes on both the patched and unpatched binary, since it asserts
behavior rather than speed. test/powershell/Language and test/powershell/engine (6764
tests) show no regressions against an unpatched baseline build of the same commit.

The condition as Regex fast path in the same method is deliberately left untouched, so this
does not interact with #8946.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• [ x] Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge. If this PR is a work in progress, please open this as a Draft Pull Request and mark it as Ready to Review when it is ready to merge.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed