feat: allow missing default attribute for let tag

[DylanPiercey]

Description

The <let> tag was needlessly restricted to require a default attribute, so if you had <let/x/> it'd error instead of just initializing as undefined.

This PR updates it to no longer error in that case.

Checklist:

• [ ] I have updated/added documentation affected by my changes.
• [x] I have added tests to cover my changes.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 0d4e85c6c1f6e41550b123372da757a81e7a428a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------------------- | ----- | | @marko/tags-api-preview | Patch |

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[codecov[bot]]

Codecov Report

Merging #22 (0d4e85c) into main (c396076) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   95.99%   95.99%           
=======================================
  Files          62       62           
  Lines        1023     1023           
  Branches      329      329           
=======================================
  Hits          982      982           
  Misses         14       14           
  Partials       27       27           

Impacted Files	Coverage Δ
src/components/let/translate.ts	97.50% <100.00%> (ø)
src/util/deep-freeze/transform.ts	100.00% <100.00%> (ø)

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