Add support for verbatim template strings

[wszqkzqk]

Verbatim template strings will be added to vala 0.58.0.

Example:

var b = 123456;
var c = 111111;
var a = @"""$b\$c\"""; // 123456\111111\

[Prince781]

Requires updating the tm grammar file. Also needs to be supported in tree-sitter-vala. VLS will automatically support this as it uses libvala.