Add font size controls (A- A+) to the Markdown viewer/editor

[AstralGuardian2]

When I open a md file in a mobile browser, the text is too small to read and edit comfortably. Sometimes on a desktop browser :( qwq

So I added A- / A+ to adjust font size.

in edit(basic):

In easymde:

• a small UI improvement;
• In order to avoid alignment issues, I change font-size, not zoom. The font size is synchronized between the viewer and editor.
• not stored in localStorage; only applies to the current page.
• Each click changes the font size by 2px, with limits.

What do you think of this UI and functionality?