[Theme] Delay watcher events until file is fully written

[frandiox]

WHY are these changes introduced?

A bit of a shot in the dark but might help in #4764

Tailwind seems to be clearing the output file first, then building its CSS, then writing the actual output content. In this scenario, we'd be emitting an event for the empty file first, then another one every time the file is written (could be in multiple chunks, unsure). Perhaps the latter events are lost or throttled down the line.

WHAT is this pull request doing?

This delays the watcher events until a safe period of time passes without file size changes. It delays all events, but it's a short period that might not be very noticeable.

The alternative would be adding some sort of debouncing logic before calling bulkUploadThemeAssets, or canceling in-flight fetch calls and sending new again as they come. These approaches seemed more complex that delaying the watcher events directly.

If the current solutions works in #4764, we might want to set a sensible default (e.g. 400ms) or introduce a flag / env variable to customize this for situations where there are big files.

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

[frandiox]

/snapit

[github-actions]

🫰✨ Thanks @frandiox! Your snapshot has been published to npm.

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[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	72.08% (+0.01% 🔼)	8364/11603
🟡	Branches	68.56% (+0.02% 🔼)	4054/5913
🟡	Functions	71.38%	2195/3075
🟡	Lines	72.45% (+0.01% 🔼)	7908/10915

Test suite run success

1894 tests passing in 866 suites.

Report generated by 🧪jest coverage report action from df469f1