Fixes issue when cssRules are inaccessilbe (#384)

When cssRules are inaccessible in a style sheet, getCSSRules
will throw an exception trying to read them, and then the
catch handler will fetch the css. But, when it goes to
insert the rule, it has a bug where it uses sheet.cssRules.length,
which causes an exception again and the rules are lost.

This results in the error:

Error inlining remote css file SecurityError: Failed to read the 'cssRules' property from 'CSSStyleSheet': Cannot access rules

This is referenced in a few issues such as issue #49.

src/embed-webfonts.ts

@@ -151,7 +151,7 @@ async function getCSSRules(
               .then((metadata) => embedFonts(metadata, options))
               .then((cssText) =>
                 parseCSS(cssText).forEach((rule) => {
-                  inline.insertRule(rule, sheet.cssRules.length)
+                  inline.insertRule(rule, inline.cssRules.length)
                 }),
               )
               .catch((err: unknown) => {