[string_scanner] Extract LineScannerMixin and deduplicate eager scann…

[amanmaurya92]

Description

This PR refactors LineScanner and EagerSpanScanner in package:string_scanner to share eager line and column tracking implementation via LineScannerMixin:

• Deduplication: Extracted shared line/column state tracking, position= setter logic, _adjustLineAndColumn, scanChar, readChar, and scan into mixin LineScannerMixin on StringScanner.
• EagerSpanScanner cleanup: Updated EagerSpanScanner to use with LineScannerMixin, eliminating ~120 lines of duplicate regex-based state tracking and removing the legacy TODO(nweiz) tracking sdk#23770.
• Zero Breaking Changes: Preserves existing class contracts, behavior, and public API compatibility.
• Changelog: Added an entry in pkgs/string_scanner/CHANGELOG.md.

Verification

• dart analyze: 0 issues found.
• dart test: All 185 tests passed across line_scanner, span_scanner, string_scanner, and error_test.
• dart format: Code formatted according to Dart style guide.

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Code Review

This pull request refactors the line and column tracking logic by introducing a new LineScannerMixin on StringScanner. This mixin is then used by both LineScanner and EagerSpanScanner, successfully eliminating a significant amount of duplicated code. The feedback suggests adding a code example to the documentation of the newly introduced public LineScannerMixin to comply with the repository's style guide.