Allow text size configuration in text panels

[gordlin]

Related Item(s)

Issue #391

Changes

• [FEATURE] Add a toolbar item in the text editor to modify text size
• [FIX] Change existing toolbar buttons to select content inside the markdown tag(s), so it can be easily changed

Notes

I chose the <span> method as it allows for more granular modifications of text size, where you could edit a single sentence, a few paragraphs, or the whole document. Using customStyles would mean you could only modify the entire slide at once.

Testing

Steps:

1. Go to any product (e.g. 00000000-0000-0000-0000-000000000000)
2. Go to any text editor.
3. Add some text, then highlight it. Go to the 'text size' option (Aa) and change the text size. It should show up in the preview.
4. Unselect all text, then change text size. <span>'s should wrap a new string "resize", with the new font size.
5. Try using Superscript or any other Storylines-custom toolbar items. Notice that the content text (here, the text between the tags) is selected, making it easy to modify. Before, it would leave the cursor after the final tag.

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This change is 

[github-actions]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/storylines-editor/text-size

[RyanCoulsonCA]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @gordlin)

[yileifeng]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @gordlin)

[IshavSohal]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @gordlin)

[szczz]

Couldn't directly test as the build is out of date. Just did a code sanity check.

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @gordlin)