fix: ensure leading slash in generated URL

[benmccann]

• 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

• What is the new behavior (if this is a feature change)? fixes a regression from support Vite's base config when using the dev server #489

• Does this PR introduce a breaking change? no

• Other information:

[changeset-bot]

🦋 Changeset detected

Latest commit: 8de2c52

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 #495 (8de2c52) into main (0da387f) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #495   +/-   ##
=======================================
  Coverage   96.94%   96.94%           
=======================================
  Files          32       32           
  Lines        1145     1145           
  Branches      235      235           
=======================================
  Hits         1110     1110           
  Misses         35       35           

Flag	Coverage Δ
imagetools-core	98.72% <ø> (ø)
rollup-plugin-imagetools	99.25% <ø> (ø)
vite-imagetools	84.66% <0.00%> (ø)

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

Impacted Files	Coverage Δ
packages/vite/src/index.ts	84.66% <0.00%> (ø)

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