Gate ES2025 regex syntax (duplicate named groups, pattern modifiers) behind target

[sumukhl17-lgtm]

Fixes #63682

Both duplicate named capturing groups and pattern modifier syntax
((?i:...)) were being parsed and validated correctly, but weren't
gated to ES2025+ the way named capturing groups are gated to ES2018
and the v flag is gated to ES2024.

• scanGroupName now errors (TS18063) when a capturing group name
  is reused across mutually-exclusive alternatives and the target
  is below ES2025 — this sits alongside the existing mutual-exclusivity
  check (TS1515) rather than replacing it.
• The pattern-modifier scan site in the (? switch now errors
  (TS18062) when modifier text is present and the target is below
  ES2025 — bare non-capturing groups ((?:...)) are unaffected since
  no modifier characters are scanned in that case.

Added a new conformance test
(tests/cases/conformance/es2025/regexPatternModifiersAndDuplicateNamedGroups.ts)
targeting es2022 and es2025 to show the gate firing/not firing.
Also updated the existing regularExpressionScanning.ts baselines
(es5, es2015), which already had test cases for both features and
now correctly produce the new errors.

I used AI assistance while working through this fix and
reviewed/tested the result myself before opening this PR.

[Copilot]

Pull request overview

This PR ensures ES2025 regular-expression syntax is correctly gated by target, aligning duplicate named capturing groups and pattern-modifier groups ((?i:...)) with how other regex features are version-checked in the scanner.

Changes:

• Adds an ES2025-only gate (TS18062) for pattern modifier syntax when modifier text is present.
• Adds an ES2025-only gate (TS18063) for duplicate named capturing groups that are otherwise valid due to mutual exclusivity.
• Adds a new conformance test covering es2022 vs es2025, and updates existing regex scanning baselines accordingly.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/cases/conformance/es2025/regexPatternModifiersAndDuplicateNamedGroups.ts	New multi-target conformance case demonstrating errors at es2022 and acceptance at es2025.
tests/baselines/reference/regularExpressionScanning(target=es5).errors.txt	Updates expected diagnostics to include TS18062/TS18063 under older targets.
tests/baselines/reference/regularExpressionScanning(target=es2015).errors.txt	Updates expected diagnostics to include TS18062/TS18063 under older targets.
src/compiler/scanner.ts	Implements ES2025 target gating for pattern modifiers and duplicate named groups (mutually exclusive case).
src/compiler/diagnosticMessages.json	Introduces TS18062 and TS18063 diagnostic messages.

[sumukhl17-lgtm]

@microsoft-github-policy-service agree

[MartinJohns]

@sumukhl17-lgtm You implemented the change in the wrong repository. See https://github.com/microsoft/TypeScript/blob/main/CONTRIBUTING.md

[RyanCavanaugh]

The TypeScript repo is closed for development; PR should be in the typescript-go repo. Please see the top section of CONTRIBUTING.md and pinned issue #62963.