FSharpSyntax: Allow lowercase hex characters

Some more text to test against
let trim (str:String) =
        str.Split( [| '\n' ; '\r' |], StringSplitOptions.RemoveEmptyEntries)

'\u1234'

let bep = '\U1A343214'

'a'
let ``shouldnt catch`` = '\uwert' <-- not legal
let var' = var + 1 <-- Shouldn't mess with var'
'\uabcd' <-- legal
'\uABCD' <-- legal

's';'b';'c'