browser: address `frameLocator` PR review comments

[pkalsi97]

What?

Address pending review comments from PR #5075

• Use ErrElementNotVisible sentinel error in stepIntoFrame
• Add newPageWithIFrameSrcdoc test helper utility
• Move FrameLocator type to locator.go for better organization

Why?

Follow-up from Issue #5129

Checklist

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

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• [ ] I have added the correct milestone and labels to the PR.
• [ ] I have updated the release notes: link
• [ ] I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• [ ] I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

PR: #5075 Closes #5129