[Diff] Scope trailing spaces

[deathaxe]

Resolves #1304

This PR...

1. sets syntax specific setting "draw_white_space": ["trailing_all"] so trailing spaces are rendered by default.
2. scopes trailing spaces meta.whitespace.trailing in inserted/removed/modified lines, so color schemes can address them and possibly apply background color.

Example:

Color Scheme Rules

{
	"scope": "markup.changed.diff meta.whitespace",
	"background": "color(var(orange) alpha(0.15))",
},
{
	"scope": "markup.deleted.diff meta.whitespace",
	"background": "color(var(red2) alpha(0.15))",
},
{
	"scope": "markup.inserted.diff meta.whitespace",
	"background": "color(var(green2) alpha(0.15))",
},

[keith-hall]

1. sets syntax specific setting "draw_white_space": ["trailing_all"] so trailing spaces are rendered by default.

Is this additive with the user preferences? If I have my draw white space setting set to all, having this syntax specific setting won't "undo"/hide/override my array, just add to it? Or will it only draw (perhaps selected and) trailing whitespace? (If I were at a computer I'd check it myself...)

[deathaxe]

Well, syntax specific settings take precedence over user settings, so "trailing_all" would win in diffs.

EDIT: Maybe better to add scopes, only, as displaying leading spaces may also be desirable.