Analogous to other ApiItem mixins, the new ApiAbstractMixin
adds 'isAbstract' as a property aspect. It is mixed into
ApiClass and ApiMethod, the only models that may have an
'abstract' modifier. ApiModelGenerator detects the modifier
and adds it to the models.
This change updates all generated *.api.json files to
include the new "isAbstract" property.

Generate "abstract" if an API item has the ApiAbstractMixin
and 'isAbstract' is true.
So far, this only works for methods, since the classes
overview table does not have a "Modifiers" column.

Split classes into abstract and non-abstract classes
and add a separate overview table for "Abstract Classes".
The alternative would be to add a "Modifiers" column to
the existing "Classes" overview table.

…also for ApiProperty

When adding 'abstract' for classes and methods, @zelliott
mentioned that properties can have this modifier, too.
Thus, ApiProperty must be extended just like ApiMethod and
ApiClass, and ApiModelGenerator must extract the 'abstract'
modifier into the ApiProperty model.
Expected test results change accordingly.

…ract-modifier

# Conflicts:
#	apps/api-extractor/src/generators/ApiModelGenerator.ts

The support to track and render 'abstract' modifiers on
classes, methods, and properties affects api-extractor-model,
api-extractor, and api-documenter.

Unlike other languages like Java, in TypeScript, the order of
modifiers is defined strictly. Currently, it looks like api-documenter
uses alphabetical order, which is not the correct TypeScript modifier
order. It seems the correct order is

(`public` | `protected` | `private`)? (`static`? | `abstract`?) `override`? `readonly`?

No member can be `static` and `abstract` at the same time, so the
order of these two does not matter. api-documenter currently only
uses `protected`, `static`, `abstract` (new in this PR) and `readonly`,
so `public` (default!), `private` (no API!) and `override` (maybe
interesting for navigation, but not as a modifier) do not apply.

Added a code comment about the order and moved `readonly` to the
last position so that the overall modifier output order now matches
TypeScript syntax.

 The schema version was updated from 1010 to 1011 in 2088fd3,
 so this changed should be squashed with 2088fd3.

…& property

Add a simple abstract class with an abstract method and an
abstract property at
build-tests/api-documenter-tests/src/AbstractClass.ts,
re-export it in the corresponding index.ts
and rebuild build-tests/api-documenter-tests.
The updated report files look good. Especially note the
"Modifiers" column of both tables in
api-documenter-test.abstractclass.md
now containing the "abstract" entry.

…ort-abstract-modifier