src: Set exe_name to 'newlang' unconditionally

main: Allow passing exe_name for better looking output

src: Move run.py to __main__.py ...

This allows running NewLang as a Python module.

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.

parse: Rename oldparse back to parse ...

This is the primary actually working parser, not an old one.

parse2: Move all new parser code to the parse2 namespace ...

This is a bit of a messy mass rename but it seems to work and makes things
clearer in preparation for a new parse module.

This also renames the NEWLANG_NEW_PARSE test environment variable to
NEWLANG_PARSE2.

parse: Take parse_file out of Parser wrapper

parse: Drop NoteSkipper wrapper class

parse: Drop enum class in favor of DIY method ...

This is slightly sketchy but should be enough to get MicroPython working
with the new parser.

src: Use UnicodeError instead of UnicodeDecodeError ...

MicroPython doesn't support UnicodeDecodeError, just UnicodeError.

main: Use new parser if NEWLANG_NEW_PARSE is set ...

This is not intended to be actually used outside testing compatibility
with micropython as we don't actually have tests on it yet.

parse: Create messages from ParseTasks

parse: Add filename to error messages

parse: Create messages from ParseErrors

parse: Rename ParseError messages to ParserError

parse: Don't generate message ids, use a table ...

I've decided that all translation strings should be used in source code
as-is without any kind of building.

parse: Add function to fully format an error

parse: Add function to format a ParseContext for display

parse: Add function to format a ParseErrorException for display

src: Check for None in all __eq__ methods ...

This lets us finally compare parse error exceptions and contexts of
different shapes.

main: Put executable name in usage

main: Add run_file function for REPL use ...

This should help using NewLang from micropython.

log: Rely on main for the getenv call ...

micropython on the Pi Pico for some reason doesn't support getenv, so
try and push this in to main where we can decide to use it.

main: Remove unused nowait_main function

port: Add micropython-compatible atexit wrapper

run: Use run.py to run src/main.py ...

This completely removes the weird logic for editing paths at the expense
of some more code.

i18n: Add mutation tests

i18n: Add new module containing a Message class ...

This Message class will be used to format localized text later.

parse: Have parse run parse_file

parse: Actually add parser code for real this time I promise