Add CLI for the backend

[a-recknagel]

If you want to know what a stenotype expression evaluates to, adding it to a source file and printing that variable's __annotation__ is a cumbersome workflow. We should have a CLI that works like this once stenotype was installed:

$ stenotype "bool or ?int"
typing.Union[bool, typing.Optional[int]]
$ stenotype "some invalid stenotype expression"
stenotype.ParseError: Failed in some way, maybe some useful info here: ... 

It would just call the backend, skip any assertion steps (which shouldn't be part of the backend anyway), and try to parse the expression into a standard type annotation using the typing module and its syntax.

• multiple arguments are interpreted as multiple lines following each other, useful for TypeVars that need to be set up
• there should be an optional argument like -m <path to file> that will load the file in order to look up class names that might be referenced in the stenotype expression
• other possible flags, that should be considered for the implementation already but are not necessary yet:
  • --shorten or -s, inverts the parser and turns a verbose type annotation into a short one
  • --checks or -c disables the check-skipping step, i.e. the code will be written exactly as it would be in the source file, and something like var: "?bool" # stenotype: no would not be translated. This would be useful to troubleshoot the is-this-recognized-as-stenotype rules.

---

• [x] add stub backend module that pretends to be working already
• [x] write cli module that runs the commands as specified
• [x] write console_script section that installs the cli

[a-recknagel]

If you want to test the cli, checkout the branch from #18 and run poetry install again (new entrypoints need to be explicitly installed). After that, stenotype --help should work.