fix(sirv): skip req._decode shortcuts; decode manually

[lukeed]

With lukeed/polka#175, there's no longer a toDecode parameter, so sirv must do it manually.

Also, through vitejs/vite#4728 discussion, relying on existing req.path or req._decoded mutations could cause issues downstream, depending on how (or how often) sirv was invoked. To deal with this, sirv invokes @polka/url every request for consistency, which is fine because the parser is so fast.

[codecov-commenter]

Codecov Report

Merging #118 (4b417e1) into master (435473f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #118   +/-   ##
=======================================
  Coverage   62.23%   62.23%           
=======================================
  Files           2        2           
  Lines         143      143           
=======================================
  Hits           89       89           
  Misses         54       54           

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