Consider to make toolbar icon monochrome

[yhatt]

According to VS Code extension guideline recently published (https://code.visualstudio.com/api/references/extension-guidelines), Marp's colored icon is going against the guideline of editor actions clearly.

❌ Don't

• Add more than one icon
• Add custom colors
• Use emojis

I've already made the variation of icon in #105, and we have taken the colored icon because it may make finding out Marp icon difficult in the drastic case of changing from the brand logo to monochrome outline. But now we can apply step-by-step updation of the outline from colored to monotone.

	Current	Monochromed outline
Dark theme
Light theme