Use `SizeParser` to correctly parse and validate `windows.PageFileFix` fields

[mdedetrich]

In #146 we implemented windows.PageFileFix using bare String's which means its possible to put in incorrect values. Ideally we should use https://github.com/sbt/sbt/blob/973cd63e0b6bfe0d06d364f314f391a08a77d38b/internal/util-complete/src/main/scala/sbt/internal/util/complete/SizeParser.scala however its (largely) blocked by #147 (its possible to work around this but its not nice).