Add Get more extensions link to brave://settings

[bsclifton]

Fixes brave/brave-browser#5540

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

1. Visit chrome://settings/extensions
2. Verify new Get more extensions item shows up. Looks like this:

1. Click link and verify it goes to the Chrome Web Store

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[bsclifton]

Ready for review! 😄

[petemill]

Did we make all new strings because we didn't want to re-use the existing ones from the extensions page @bsclifton ? Shouldn't we prefix the new strings resource IDs with BRAVE_?

[emerick]

LGTM

[bsclifton]

@petemill good call - I didn't see those... I can re-use one of the two strings. Will update + put BRAVE prefix

[bsclifton]

@petemill @emerick updated! 😄

[bsclifton]

All checks passed! WOW

[kjozwiak]

Quickly went through a quick check using the test case outlined under #3166 (comment) before bringing up #3229 and #3231 to @brave/uplift-approvers. Verified using 0.71.24 CR: 76.0.3809.100. Example:

• ensured that https://chrome.google.com/webstore/category/extensions?hl=en-US was being opened when clicking on the Get more extensions button via brave://settings.