build: Switch to regular Python building

main: Allow passing exe_name for better looking output

src: Move all source code to src/newlang ...

The Python ecosystem works much better if we deicide our source code
is actually a module named 'newlang'.

I've been putting off changing this for a long time, but I might as
well do it now.

build: Note about executable mode

build: Always zip creation system to Unix ...

This doesn't matter and it means we can have reproducible builds across
Windows and Linux.

build: Tidy write_file

build: Ignore hidden files

build: Replace Windows line endings with Unix ones

build: Open files as binary read-only

build: Add shebang to zipapps

build: Replace git date with 2000 ...

There's no real point having artifical differences in our zip files.

build: Rewrite to make a reproducible zip

main: Add a main function that waits ...

This is enabled in the zipapp to make life easier when the program is
run from a GUI.

build: Add script to build NewLang.pyz