Add missing heading options

[SergioEstevao]

Fixes #1261

As discussed with @iamthomasbishop on the ticket above we decided to implement the missing heading options by just adding them to the format toolbar.

Related GB PR: WordPress/gutenberg#17533

To test:

• Start the demo app
• Check that the heading block has the H4, H5, H6 buttons when selected
• Use the option and check that the HTML is correctly generated.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[etoledom]

Works great on both iOS and Android 🎉

This is how it looks in both platforms:

✅ for behavior and code.

Let's ask @iamthomasbishop about the looks of it 👍

[iamthomasbishop]

Looking mostly good, I have a couple thoughts.

The font sizes seem a bit small, esp on Android. Here's what I've got them defined as on the design side (in Figma):

• h1: 24
• h2: 22
• h3: 20
• h4: 18
• h5: 16 (same as "normal" p text)
• h6: 14

Also, as noted in this issue, the inside block content areas are a bit off. This throws off the vertical rhythm substantially on Android.

[SergioEstevao]

@iamthomasbishop I just confirmed that in iOS the sizes match you specify above.

[iamthomasbishop]

@SergioEstevao can you investigate why Android looks different? They look off in the screenshot above but I’m not sure if that’s my eyes playing tricks?

[hypest]

👋 @SergioEstevao , love to see this merged! 🎉

I wonder, do you think we could add one or more automated tests in this one? Maybe an Appium e2e test?