HTML block with classic toolbar

[knutsp]

Is your feature request related to a problem? Please describe.
When coming from WordPress 4.9.8 til 5.0, previously using the text (not visual) editor, now using the HTML block as replacement, the toolbar with formatting buttons a are gone. This is a regression for those users.

See https://core.trac.wordpress.org/ticket/45375#comment:25

Describe the solution you'd like
Add all the toolbar buttons from the classic editor, text tab, to the HTML block.
Allow registering new buttons as before.

Describe alternatives you've considered

1. Install and activate Classic Editor plugin. As this is probably phased out in 2022, it doesn't represent a permanent solution.
2. Develop a new block, "HTML with classic toolbars". Ensuring users don't face regression is, however, a core responsibility.

[youknowriad]

Some context here #9807 (comment)

I think the second option you propose is better, whether it's going to be done in Core or in a plugin remains to be seen.