Restore test coverage for “x-parser.js”.

In a prior change set, we changed our `Map` error mappings to `switch`
error mappings so that our code coverage tests would ensure that we are
able to prove that each error is possible to be shown — otherwise, why
keep them.

This change set was kept separate since it’s all about coverage.

Note that some of the regular expressions have changed — nearly all of
that is due to rewriting like `\s\n` as just `\s` since `\n` is one of
the characters matched by the `\s` set.

Author: theengineear

test/index.js

@@ -7,7 +7,7 @@ import '../x-template.js';
 
 // Set a high bar for code coverage!
 coverage(new URL('../x-element.js', import.meta.url).href, 100);
-coverage(new URL('../x-parser.js', import.meta.url).href, 97); // TODO: Get back to 100%!
+coverage(new URL('../x-parser.js', import.meta.url).href, 100);
 coverage(new URL('../x-template.js', import.meta.url).href, 100);
 
 test('./test-parser.html');