Implement file.replace

[hulto]

Implement Eldritch's file.replace function, which should exhibit the following behaviour:

• If the file does exists this method should error.
• It should take a regular expression using the Regex library and a value string to replace matching with.
• It should be implemented to be cross platform (e.g. Windows support). If an invalid path is supplied for the current operating system, an error should be returned.