scanner: check unknown escape sequence (fix #19131)

[yuyi98]

This PR check unknown escape sequence (fix #19131).

• Check unknown escape sequence.
• Add test.

fn main() {
	println('¯\_(ツ)_/¯')
}

PS D:\Test\v\tt1> v run .
tt1.v:2:14: error: `_` unknown escape sequence
    1 | fn main() {
    2 |     println('¯\_(ツ)_/¯')
      |                ^
    3 | }