fix: percent-encode page URLs built from file paths

A file named Müsli.html was indexed with the URL /Müsli.html, which only
resolves if the host happens to serve the same byte sequence the index was
built from. Encode the URL that build_url derives from the file path using
the URL spec's path percent-encode set, leaving / and % alone so path
separators and already-encoded filenames are unaffected.

URLs supplied by the caller (data-pagefind-meta="url:...", addCustomRecord)
do not go through build_url and are untouched.

Fixes #619

Cargo.lock

@@ -2303,6 +2303,7 @@ dependencies = [
  "minifier",
  "pagefind_stem 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "path-slash",
+ "percent-encoding",
  "portpicker",
  "rayon",
  "regex",