Set base typographic rhythm

[hypest]

Fixes #550

Adds support in ReactAztecText (Android only for now) for the lineHeight property. At the same time, setting the default fontSize to 16(px) and default lineHeight to 26(px).

Note: Since this PR changes the defaults for the RichText component, blocks that depend on it might change as well. For example the List block, Heading block and others.

Gutenberg PR: WordPress/gutenberg#23079

WPAndroid PR for testing this with a prebuilt APK: wordpress-mobile/WordPress-Android#12156

Keeping this PR as a draft until we decide whether to wait for the iOS side implementation to happen on this PR, or not.

To test:

1. Run the demo app and have a paragraph block with a handful of lines of text
2. Check that the line spacing is around 1.6x

Screenshots

Before	After

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

Wanna run full suite of Android and iOS UI tests? Click here and 'Approve' CI job!

[etoledom]

I'm implementing setting line height on iOS, it works but I haven't pushed since I'm seeing a few problems with it.

In general terms, it's good to let the system take care of line spacing, and set it manually just as exceptions when it's required.

In this case, having a font size that is smaller than the manually set line height is giving us some troubles on List Block and Preformated block:

Before	After

The text inside a bigger line will fall to the bottom, having extra space at the top that is visible at the top of the preformatted block, and makes the bullets unaligned on list blocks.

cc @iamthomasbishop @SergioEstevao

[hypest]

Thank you @etoledom !

Closing this PR until we go back working on #550 as it has gone stale.