QQmlJS::Lexer::scanNumber: Use a QVarLengthArray to avoid heap allocations.

This shaved the time of this function in half on an (admittedly very morbid)
testcase of mine, from 161ms to 72ms total.

Change-Id: I88ad8d4ad4791dc4e7eadb9af9265ff02fe5376f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>