Warn on common directive typos

[jcollins-g]

Sometimes people don't get the case right for dartdoc directives or otherwise make minor errors. We could give more help to the user to make it clear what's going wrong.

[srawlins]

We could issue warnings for the following:

• any unknown directive name /\{@[a-z].*\}
• wrong case for name ({@YouTube}, @{inject-HTML})
• wrong punctuation for name (@{injecthtml}, @{end-template})
• directive with no trailing }, which may span multiple lines
• @example with src foo.dart.md when foo.dart.md.md doesn't exist; @example with src foo.dart when foo.dart.md doesn't exist, but foo.dart does.
• {@tool} with no {@end-tool}, etc.