Forms: add "source" to hidden field value

[simison]

Adds just UI for hidden field's sources as an experiment. We likely should pick just couple to implement for ease.

A good set could be:

• Manual
• URL param
• Page/post link

Needs some styling work to make line-height work.

Proposed changes:

Other information:

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Testing instructions:

• Go to '..'

[github-actions]

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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/blocks/field-hidden/edit.js	0/16 (0.00%)	0.00%	7 💔

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}