fix(playground): windows production build

[sibbng]

Resolved path's \u part was causing issue on Windows.

vite v2.6.13 building for production...
✓ 2 modules transformed.
[rollup-plugin-dynamic-import-variables] Bad character escape sequence (3:26)   
file: C:\Users\sib\dev\unocss\playground\__play.html:3:26
error during build:
SyntaxError: Bad character escape sequence (3:26)
    at Parser.pp$5.raise (C:\Users\sib\dev\unocss\node_modules\.pnpm\rollup@2.58.0\node_modules\rollup\dist\shared\rollup.js:19495:13)

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[sibbng]

I'll make PR at vite repository instead.