feat: allow async output formats

[zerodevx]

• Quick Checklist

• I have read the contributing guidelines
• I have written new tests, as applicable (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• I have added a changeset, if applicable

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Fixes #463 and #369 (comment), since it's possible that output format ops may be async.

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this
  PR?)

Nope.

• Other information:

I'm still facing an unrelated test failure on a brand new imagetools setup as described in #506. Happy to rebase if that get resolved.

[changeset-bot]

🦋 Changeset detected

Latest commit: 8786522

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

This PR includes changesets to release 1 package

Name	Type
vite-imagetools	Patch

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[codecov]

Codecov Report

Merging #507 (3baa612) into main (e1ea30e) will increase coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 3baa612 differs from pull request most recent head 8786522. Consider uploading reports for the commit 8786522 to get more accurate results

@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
+ Coverage   96.95%   97.04%   +0.08%     
==========================================
  Files          33       33              
  Lines        1149     1149              
  Branches      238      239       +1     
==========================================
+ Hits         1114     1115       +1     
+ Misses         35       34       -1     

Flag	Coverage Δ
imagetools-core	98.72% <ø> (ø)
rollup-plugin-imagetools	99.25% <ø> (ø)
vite-imagetools	85.71% <100.00%> (+0.64%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/vite/src/index.ts	85.43% <100.00%> (+0.66%)	⬆️

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