Update Sobek and expand TC39 coverage

[mstoykov]

What?

• Update github.com/grafana/sobek to 4698bc773ae7.
• Update and vendor github.com/dlclark/regexp2/v2 v2.7.2.
• Remove 31 TC39 expected failures fixed by the Sobek update.
• Enable TC39 coverage for:
  • uint8array-base64
  • set-methods
  • String.prototype.replaceAll
  • logical-assignment-operators
  • regexp-named-groups
• Record the six remaining named-group edge-case failures instead of blocklisting the entire feature.

Why?

The updated Sobek version adds new JavaScript functionality and fixes several existing conformance failures. Enabling the corresponding Test262 suites ensures that k6 exercises these features and detects future regressions.

Testing

• go mod verify
• go test -count=1 ./internal/js/tc39
• go test -count=1 ./internal/js/...
  • All non-browser packages passed.
  • Browser tests could not run successfully because Chromium access is restricted by the local sandbox.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Related PR(s)/Issue(s)

• Upstream Uint8Array base64 support: Implement Uint8Array.fromBase64, Uint8Array.prototype.toBase64, Uint8Array.prototype.setFromBase64 dop251/goja#731
• Upstream ES2025 Set methods: Implement ES2025 Set methods dop251/goja#732