regex escape sequence support

[AStenbaek]

At the moment regex is checked with the same function as strings.
This results in the compiler rejecting regular expressions that are valid in the Java implementation, the system is based on,
due to some escape sequences being illegal in strings.

As an example, the following program is currently rejected:

def main(): Unit \ IO = Regex.isMatch(regex"\w", "W") |> println

The issues stems from the following code:

flix/main/src/ca/uwaterloo/flix/language/phase/Weeder.scala

Lines 1746 to 1752 in dcb8dba

	case ParsedAst.Literal.Regex(sp1, chars, sp2) =>
	visitCharSeq(chars).flatMap {
	case s => toRegexPattern(s, mkSL(sp1, sp2)).map {
	case pat => Ast.Constant.Regex(pat)
	}
	}
	}

The weeder should be updated to allow escape sequences from: https://docs.oracle.com/en%2Fjava%2Fjavase%2F21%2Fdocs%2Fapi%2F%2F/java.base/java/util/regex/Pattern.html#cg