tox: Allow passing arguments to mumut

Jenkinsfile: Remove support for Jenkins ...

The Linux build is broken because containers don't have root access in the container.
The Windows build is broken because my tox install is not in the system PATH.

These are definitely fixable, but I don't have the energy to go through the
winding maze of Jenkins documentation, the web interface and get told all my
plugins are orphaned and full of security holes. It's too much.

So I'm just dropping Jenkins support for now.

tox: Fix mutmut

Jenkinsfile: Try to fix Linux build

env: Dump environment system

tox: Run check commands by default

tox: Add support for tox file ...

This is initial support for using tox to replace my env.sh files.

Benefits over the env.sh files:

- Better testing environments
- More familiar tooling

Cons over the env.sh files:

- Lots of duplication

gitignore: Specify pycache is a directory

env: Fix mutmut paths

Jenkinsfile: Fix build

gitignore: Ignore build/ directory

README: Install and run guide

src: Set exe_name to 'newlang' unconditionally

build: Switch to regular Python building

requirements: Drop unused grip dependency ...

Apparently I forgot to add grip support to env.sh and only noticed now.
I'm not using it so just remove it.

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.

env: Clean up run.py also

env: Pass --use-pep517 to pip ...

I don't fully understand this, but we need to do this now.

More reading:

https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html

I'm not sure exactly

env: Fix and simplify mutmut mutating ...

Instead of passing individual files, just parse the parse2 directory.
Make sure to do this on Windows too.

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.

tests: Add more test_note documentation

tests: Add more clear_notes documentation

parse: Take parse_file out of Parser wrapper

parse: Drop NoteSkipper wrapper class

tests: Fix invalid directive comments

tests: Check clear_notes error passthrough

tests: Replace file parsing mock with static directives