Allow theming the diff view's diagonal fill with diffEditor.diagonalFill

[robertrossmann]

Themes can now customise the colour and opacity of the diff editor's diagonal fill.

I have replaced the diagonal fill image (png) with a CSS version which allows us to customise the fill's colour. Then I added a new colour contribution point, diffEditor.diagonalFill with default values very closely matching the original look. The light themes, however, look considerably better with this new version (screenshots attached).

Benefits

• we can style the stripes' colour
• themes can provide their own colour
• the stripes remain sharp even at high editor zoom levels (the image-based stripes start to be visibly blurred at zoom level 3-4).
• Dunno, maybe it now requires less memory to render...? 🤷‍♂️

Screenshots

Click on a screenshot to see the full image.

old	new	diffEditor.diagonalFill
	remedy dark
	remedy bright
		❌
	Light+
		❌
	Abyss
		❌
	Kimbie Dark
		❌
	Solarized Dark
		❌

[robertrossmann]

I just rebased this on top of current master. Is there anything I can do to get this merged in? 🙏 ❤️ @alexdima

Thanks in advance!

[alexdima]

This is great! Thank you!