Add HeadingSelectControl component

[tfrommen]

The HeadingSelectControl component allows for choosing one of a pre-defined list of heading levels.
It is intended to be used for blocks or plugin sidebars where some text element needs to be rendered as a user-defined heading.
The component wraps a regular SelectControl component.

HeadingSelectControl component.

HeadingSelectControl component with custom min and max value specified.

[tomjn]

Core has icons and options for H1/2/3/etc does it make sense to use those? I can foresee a toggle with these as buttons next to each other would be nice to have and avoid the need for translations

[tfrommen]

Core has icons and options for H1/2/3/etc does it make sense to use those? I can foresee a toggle with these as buttons next to each other would be nice to have and avoid the need for translations

Yep, I know the SVGs. But I don't think I'd want to use them here.

Core uses the icons in a single Toolbar Button. This component, however, is a sidebar control (because you might want/have to use it multiple times for a specific, more complex block). I don't really like a button group with those icons, especially given they would wrap into two lines if you used multiple/all heading levels.

Also, the icons only work/exist for the default heading levels. This component also allows for using heading levels higher than 6, which will then be used to indicate some special form of a subheading or super paragraph or whatever. The project this component is coming from uses a total of 7 heading levels.

I just added a commit yesterday that allows for passing a custom function to create the label text for a given heading level. Maybe this better explains/justifies the way the component is set up (as a dropdown of text options).

Do you have any (other) thoughts on the current implemenation, @tomjn?