feat(material-experimental/mdc-select): implement MDC-based mat-select

[crisbeto]

Implements mat-select on top of MDC Web's styles. The new select is functionally identical to the current one since both are implemented on top of the same base class, but they have the following differences:

• The logic that tries to position the selected option on top of the trigger has been removed from the MDC version. Now we only position the panel above or below, depending on the available space.
• Since we don't have to deal with the panel positioning logic anymore, we can now allow the text of mat-option to wrap to multiple lines inside the MDC version.
• The animations of the MDC version are simplified to mimic the ones from mat-menu, because the previous animations were tightly coupled to the old overlay positioning logic.

Aside from the new select, this PR includes the following fixes:

• The MatOptionSelectionChange.source is now typed to the generic _MatOptionBase rather than the non-MDC implementation. This was an oversight when the base class was first implemented.
• Uses min-height instead of height for the MDC mat-option in order to handle multi-line text better.
• Adds the mat-form-field-hide-placeholder, mat-form-field-appearance-outline and mat-form-field-appearance-fill classes which were missing from the MDC form field. They're necessary so that the select can be styled to look correctly across all supported use cases.
• The dev app for the select has been reorganized to emphasize mat-select, instead of mat-form-field with a native select. I've also cleaned up the styles so everything aligns better and is nicer to look at.

Note: I'll add the test harness as a follow-up since the PR is fairly large already.

[crisbeto]

All of these API changes are due to me moving the logic into a base class.

[crisbeto]

The feedback has been addressed.

[mmalerba]

LGTM

[mmalerba]

Please address the last comment before marking merge ready

[wagnermaciel]

• The MatOptionSelectionChange.source is now typed to the generic _MatOptionBase rather than the non-MDC implementation. This was an oversight when the base class was first implemented.

@crisbeto Is _MatSelectBase meant to be a public type? I ask because there is at least one instance internally where we had
const matSelect: MatSelect = event.source;
which would now be
const matSelect: _MatSelectBase = event.source;

[crisbeto]

@wagnermaciel does that case throw a compilation error? I would've expected it not to complain since MatSelect extends _MatSelectBase. The base class technically isn't a public API, but I used it in the event, because we reuse the same event object for both select variants. Let me know if it's blocking you and I can find a different way around the issue (probably just duplicating the event).

[wagnermaciel]

@crisbeto Yes, I ended up getting a compilation error shortly after writing that comment but I didn't get a chance to update you. It is blocking so please let me know what that line should now look like

[crisbeto]

I've reworked it to avoid the compilation error.

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