Defence-in-depth on top of the strict iframe img-src/media-src/font-src CSP
that already blocks <style>-tag fetches at the network level. The per-node
DOM walk in blockExternalResourcesOnNode only sees element attributes, so a
tracker hidden in a kept <style> block (background url(), @font-face, @import)
never passed through it.
Adds stripExternalStyleSheetCss(), wired into blockExternalResourcesOnNode for
STYLE nodes (so it's gated on shouldBlockExternal and drives the blocked-content
banner like every other vector). Decodes CSS escapes over the whole block first
so the escaped-keyword form \75\72\6C( -> url( is caught - a literal `url(`
match would miss it. Removes remote @import in both url() and bare-string forms.
