Summary:
RN Mac's implementation of the `selectable` prop on `Text` only allows selecting the entire Text component and right click to copy. This diff makes the `Text.selectable` prop on Mac allow arbitrary selection. To do this we used `NSTextView` to render the `Text` component instead of RN Mac's custom text rendering, because it has a `selectable` property which gives us the behavior we want.

We also allow adding custom menu items in the context menu.

Note the change to RNTesterPage.js was required to fix microsoft#754.

Test Plan:
See test plan of D27250072 for integration to Zeratul.

Confirmed text selection works in RNTester Text example:
{F588619781}

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Also I went to RNTester Text examples and did an image diff comparison before and after these changes (differences are in pink):

{F588602710}
- The font smoothing isn't something we need

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{F588602715}
- The examples with images are different because they load random images
- The pink background on "With size and background color" isn't a difference, the background color is pink in code

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{F588602706}
- The <TextInput multiline/> example has an off by 1 pixel difference that wasn't trivial to fix and doesn't seem significant enough to investigate

Reviewers: skyle, ericroz

Reviewed By: skyle

Subscribers: eliwhite

Differential Revision: https://phabricator.intern.facebook.com/D27484533

Tasks: T83817888

Signature: 27484533:1617928003:6c1c60a15db8ef3551aafe22229fafc9fea0053e

# Conflicts:
#	Libraries/Text/Text/RCTTextView.m
#	React/Base/RCTTouchHandler.h
#	React/Base/RCTTouchHandler.m

Summary:
This fixes a regression introduced by D29340382 since the `contentView` of the window was changed to the `RCTRootView` instance. The problem isn't there though, but is due to now the `contentView` having flipped geometry. The `hitTest:` method expects coordinates in the superview's coordinate space:

> A point that is in the coordinate system of the view’s superview, not of the view itself.

Also see how `RCTTouchHandler` also calls `convertPoint:` on the `superview` before passing to `hitTest:` for the same reason: https://fburl.com/diffusion/krx4lxao

Test Plan: 
{F628902534}


Reviewers: lyahdav

Reviewed By: lyahdav

Subscribers: eliwhite

Differential Revision: https://phabricator.intern.facebook.com/D29469639

Tasks: T94420821

Signature: 29469639:1625001662:97028699aee404282c83e35cd66f6308bc793a2a

…ext-selection

…os into text-selection

… macOS