Add write(), read(), read_all(), join()

[healeycodes]

The big i/o update.

Add via builtins:

• write(file_path: string, data: (string | number): nil.
• read(file_path: string, read_function: (chunk: string) => nil): nil.
• join(x: string, string): string
• join(x: list, list): list

Add via stdlib:

• read_all(file_path: string): string

Tests added for all the above, including some error message tests.