Add --dev-preview flag to theme init for framework theme support

[jamesmengo]

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

[jamesmengo]

• Add theme generation commands for blocks, sections, and templates #5081 : 2 dependent PRs (#5088 , #5089 )
• Add --dev-preview flag to theme init for framework theme support #5079 👈 (View in Graphite)
• main

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[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	75.33% (-0.01% 🔻)	8837/11731
🟡	Branches	70.61% (-0.02% 🔻)	4290/6076
🟡	Functions	75.2%	2314/3077
🟡	Lines	75.87%	8354/11011

Show files with reduced coverage 🔻

St.❔	File	Statements	Branches	Functions	Lines
🟢	... / app-event-watcher.ts	93.83% (-1.23% 🔻)	86.49% (-2.7% 🔻)	90.48%	98.61%
🟢	... / ConcurrentOutput.tsx	98.36% (-1.64% 🔻)	88% (-4% 🔻)	100%	98.33% (-1.67% 🔻)

Test suite run success

1994 tests passing in 900 suites.

Report generated by 🧪jest coverage report action from 1e0708a