🏗🚀 Refactor and significantly speed up gulp check-links

[rsimha]

This PR builds on #25226 and refactors / significantly speeds up gulp check-links

Highlights:

• Moves common functionality like getFilesToCheck() into build-system/common/utils.js
• Introduces the --files and --local_changes flags (--files now supports lists of globs)
• Makes it an error to run gulp check-links on all files in the repo (there are multiple thousands of links, so we only run the link checker on files changed by a PR)
• Uses the ignorePatterns option of markdown-link-check and deletes filterAllowedLinks() (addresses 🏗 Ignore markdown templates in link checker by convention #25226 (comment))
• Caches gitDiffAddedNameOnlyMaster() in filesIntroducedByPr (significant speed up)
• Processes and prints results for each .md file as soon as they are available (better UX, also faster)
• Adds support for ignored links while reporting results (yellow dots)
• Eliminates the use of BBPromise in favor of the native interface of markdown-link-check

Usage:

# Check all .md files changed in the local branch
gulp check-links --local_changes

# Check just the files specified
gulp check-links --files "build-system/**/*.md"

Example:

Follow up to #25226

[amp-owners-bot]

Hey @estherkim, these files were changed:

• build-system/pr-check/checks.js

[alanorozco]

Are we sure we'd like to ignore build-system overall?

[rsimha]

@alanorozco Can you post the rest of your comments in one go by clicking the "Start a review" button? Posting several comments one by one generates notification spam and makes it hard to collapse / group review comments.

And thanks for reviewing!

[rsimha]

Comments addressed.

[rsimha]

Are we sure we'd like to ignore build-system overall?

Not sure what this comment means. We do include build-system/. It's also possible to explicitly check the files in there (see screenshot in description).

[kristoferbaxter]

Nice set of changes! Thanks Raghu.

[rsimha]

Thanks for the reviews, everyone. All comments addressed. Merging now, so that #25182 can be unblocked.