Rewrite parameter handling and add required named parameters for nnbd

[jcollins-g]

Fixes #2059, #2052.

The linkedParams / linkedParamsLines implementation has turned into a hot mess over time. This rewrites the parameter rendering to abstract the HTML portion of the rendering and use list items to organize the display of parameters on method pages. Minimal changes to existing tests have been made to make them pass; at some point, a better suite of tests for the new structure could replace a lot of the cut and paste of literal HTML in model_test.dart.

Screenshots:

(compare to https://api.dartlang.org/dev/2.7.0-dev.0.0/dart-core/RegExp/RegExp.html)

(compare to https://api.dartlang.org/dev/2.7.0-dev.0.0/dart-core/RegExp-class.html)

Required parameter in action:

[coveralls]

Coverage increased (+0.04%) to 93.904% when pulling d39f125 on required-param into ab52275 on master.

[devoncarew]

It looks like named params are getting a 2x indent; their indent should likely match the indent of regular, required params (for the first screenshot, {bool multiLine: false and following).

[jcollins-g]

Done, I liked the other way better visually, but this is more consistent.

[jcollins-g]

@devoncarew PTAL?

[devoncarew]

lgtm; I like the refactoring into specialized ParameterRenderer subclasses.

[jcollins-g]

@devoncarew Glad you like it. @jdkoren and I are coordinating to extract more of the rendering goop out in this manner.