Fix class helpers splitting non-ASCII whitespace class names

[sergioperezcheco]

Single class names containing non-ASCII whitespace (NBSP, EM SPACE,
NARROW NBSP) are broken by the class helpers since #552: AddClass
re-appends the name split into pieces as extra classes, RemoveClass
cannot remove it, and ToggleClass behaves wrongly, while HasClass and
the class-attribute normalization treat those characters as part of the
token.

Root cause

HTML class tokens are separated by ASCII whitespace only (WHATWG HTML
standard 2.3.7); any other character is part of the token. #552 changed
getClassesSlice from an ASCII replacer plus strings.Split to
strings.Fields, which also splits on Unicode whitespace. HasClass and
getClassesAndAttr still normalize with the ASCII-only
classTrimReplacer, so the tokenizers now disagree:

• AddClass("a\u00a0b") on a fresh element stores a\u00a0b a b
• RemoveClass("a\u00a0b") on a\u00a0b keep leaves keep (fails) and on a\u00a0b alone clears the whole attribute
• ToggleClass adds a duplicate a b instead of toggling the name
• setClasses also trimmed Unicode whitespace from attribute edges

Fix

• getClassesSlice: strings.FieldsFunc over the five ASCII whitespace
  characters (keeps the no-empty-entries behavior of perf: replace regex+Split with strings.Fields in getClassesSlice #552)
• AddClass/RemoveClass/ToggleClass entry trim and setClasses exit trim:
  trim only ASCII whitespace, preserving Unicode whitespace at attribute
  edges verbatim
• Form feed stays a separator (consistent with Recognize form feeds between HTML classes #611)

Verification

• New regressions (property_unicode_class_test.go): 35 subtests fail on
  master 0ac7142 and pass with this change; attribute round-trips are
  byte-for-byte.
• go test -race ./... passes.
• go vet ./..., go build ./..., gofmt, git diff --check pass.
• Benchmarks (darwin/arm64, Go 1.26.4, -count=3 means vs master):
  AddClass 27279 -> 28776 ns/op (+5.5%), RemoveClass 28166 -> 29047
  ns/op (+3.1%); B/op and allocs/op unchanged. FieldsFunc keeps the
  allocation win that motivated perf: replace regex+Split with strings.Fields in getClassesSlice #552.

AI-assisted contribution, disclosed per the Hermes automation workflow;
DCO sign-off included.

[sergioperezcheco]

The workflow run for head 36c04bf is marked action_required and has not started any jobs. Could a maintainer approve it so CI can run? The regression, race-test, and benchmark results are included in the PR description.

[mna]

Thanks for the fix!