Refactor extension build process to use modular build steps

[alfonso-noriega]

WHY are these changes introduced?

Fixes #0000

WHAT is this pull request doing?

How to test your changes?

Post-release steps

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[alfonso-noriega]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add a build manifest build step #6869
• Replace hosted_app_home build mode with copy_files mode #6868
• Refactor extension build process to use modular build steps #6867 👈 (View in Graphite)
• Abstract build steps to externalize the build configuration #6866
• Asset pipeline flow for hosted static app #6806 : 1 other dependent PR (#6842 )
• main

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[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	79.03%	14688/18585
🟡	Branches	73.3%	7286/9940
🟡	Functions	79.22%	3729/4707
🟡	Lines	79.39%	13891/17497

Test suite run success

3854 tests passing in 1499 suites.

Report generated by 🧪jest coverage report action from cfe3768

[github-actions]

We detected some changes at packages/*/src and there are no updates in the .changeset.
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