Include extension members on the extended type's page

[srawlins]

In this feature, we list extension members of an extended type on that extended type's page. The members are listed inline with other instance members and are cross-linked over to the extension's member pages.

• Introduce availableInstanceMethods as a list combining both instance methods declared in a container, and instance methods declared in an applicable extension. The same for availableInstanceOperators and availableInstanceFields.
• Introduce some constructors like Field.providedByExtension for members that are provided by an extension.
• Add lots of documentation clarifying where various members come from.
• We introduce a new getter on ElementType, nameWithGenericsPlain, which is not HTML-formatted, for use in HTML "title text."

Here are some screenshots:

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[srawlins]

Be excited, @dart-lang/analyzer-team, this is ready for review 😁 😀 😁 😀