Start cleaning up docs in package:js

- Remove some unnecessary details in the README. The SDK constraint
  duplicates the pubspec, the dependencies section duplicates automatic
  info on the pub site, and the contributing section duplicates generate
  Dart/SDK contributing information. The issue link is still useful
  since it fills in a template, but we should update it to include a
  label by default.
- Use "JavaScript" consistently in docs.
- Rephrase some doc comments for clarity.
- Drop `new` from examples.
- Use consistent single quotes in examples.
- Always show the `@JS()` annotation on `library` in examples.
- Move the note about factory constructors for anonymous classes to the
  annotation from the README.
- Bump the mimimum SDK to`2.0.0` since there isn't utility in claiming
  support for older SDKs.

There are stale docs remaining in the `varargs.dart` library that should
be handled separately by deleting that library.

Change-Id: Ida51d2ec3fd31210b55dc91042a0ac1cf77210b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107450
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>