Invalid string literal escape sequences should err

[mhasel]

Invalid escape sequences are interpreted as literal strings instead of producing a compile-time error:

FUNCTION main : DINT
VAR
    incomplete : STRING := '$1';   // should error: incomplete hex byte
    invalid    : STRING := '$GG';  // should error: 'G' is not a hex digit or valid IEC escape
END_VAR
    printf('%s$N', incomplete);    // prints "$1"
    printf('%s$N', invalid);       // prints "$GG"
END_FUNCTION

Expected: Compile-time error for malformed escape sequences
Actual: Silently treated as literal $ + following characters

For reference, CODESYS rejects these at compile time.

Reference table for all IEC 61131-3 escapes:

Escape	Meaning
$$	Dollar sign
$'	Single quote
$"	Double quote
$L / $l	Line feed
$N / $n	Newline (platform-dependent)
$R / $r	Carriage return
$T / $t	Tab
$P / $p	Form feed
$hh	Hex byte (STRING)
$hhhh	Hex code unit (WSTRING)