string_literal: adding \a \b \f \v escapes

[mrjbq7]

I'm not sure if #21564 is accepted, but this could be a patch that resolves it.

Fixes #21564.

[mrjbq7]

Not sure why the tests fail, if i build stage3 with this branch then this program works:

const std = @import("std");

pub fn main() !void {
    std.debug.print("\a\b\f\n", .{});
}