[UI extension sourcemaps]: Only generate with --sourcemaps option

[jamesvidler]

WHY are these changes introduced?

Generating sourcemaps for UI extensions is only useful for partners who wish to use these, and this likely won't be common. Rather than generate these for all UI extensions, allow partners to opt into the behavior.

WHAT is this pull request doing?

This adds --sourcemaps flag to the existing build and deploy commands.

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

[jamesvidler]

Closing this PR since the addition of a --sourcemaps flag would not be appropriate since it only applies to ui extensions.