fix: percent-encode page URLs built from file paths

build_url() escaped %, # and ? one by one, so a file name carrying any
other character that a URL path cannot hold - a space, a quote, an angle
bracket, a control character - was emitted raw and only resolved if the
host happened to serve the same byte sequence the index was built from.

Encode the URL that build_url derives from the file path with the URL
spec's path percent-encode set, leaving / alone so the separators between
path segments survive. % stays in the set, so a file literally named in
encoded form keeps pointing at the file that is on disk.

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",