Highlighting texts inside '~~~diff '

[liuderchi]

hope we can highlight text inside ~~~diff syntax with visual effect below:

highlight this:

~~~diff
-@import "packages/file-icons/styles/icons";
-@{pane-tab-selector},
.icon-file-directory {
	&[data-name=".git"]:before {
-		.git-icon;
+		font-family: Devicons;
+		content: "\E602";
	}
}
~~~

into this:

~~~diff
-@import "packages/file-icons/styles/icons";
-@{pane-tab-selector},
.icon-file-directory {
	&[data-name=".git"]:before {
-		.git-icon;
+		font-family: Devicons;
+		content: "\E602";
	}
}
.~~~

NOTE: currently texts anywhere have same scope

source.gfm
markup.code.diff.gfm
source.embedded.diff