Fix JS/TS template-hole line-continuation strings

[Widthdom]

Summary

• keep JS/TS template-hole string state across physical lines in StructuralLineMasker
• prevent continued string text inside template holes from closing the hole early or leaking phantom references
• add JavaScript and TypeScript regression coverage for issue JS/TS: line-continuation string inside template hole (${"foo\\\nbar"}) breaks hole scanner state #433 and update the changelog

Test plan

• dotnet test --filter "FullyQualifiedNameReferenceExtractorTests.Extract_JsTemplateHoleLineContinuationString_DoesNotCloseHoleEarlyOrLeakPhantoms|FullyQualifiedNameReferenceExtractorTests.Extract_JsTemplateHoleIfParenRegexLiteral_PreservesFollowingCall|FullyQualifiedNameReferenceExtractorTests.Extract_JsTemplateHoleReturnRegex_PreservesFollowingCalls|FullyQualifiedNameReferenceExtractorTests.Extract_JsTemplateHoleMultilineDivisionContinuation_PreservesCallAttribution|FullyQualifiedName~ReferenceExtractorTests.Extract_JsTemplateLiteral_DoesNotLeakPhantomCallsButKeepsInterpolationCalls"
• dotnet test

Closes #433.