Need a means to acquire `isActive` for use-font-size

[dvnrsn]

use-document-query-command-state exposes isActive which gives the option to change a rich text button's color if bold/italic/underline etc. are active. font-size has no such feature but could use the add.

[joeldenning]

When would isActive be false? When the rich text editor is not focused? Font size is inherently different from bold/italic/underline in that there is always a font size that is active whenever you're working in the rich text.

[dvnrsn]

This was asked in my confused state of trying to use useFontSize instead of useDocumentExecCommand for H1/H2. I have, however, addressed the formatBlock needs in this PR

#18