Allow custom python interpreter to be specified by path

[James4Ever0]

There are multiple python implementations such as pypy, jython, ironpython, rustpython etc. Also user may use different python interpreters under different conda or venv environments.

Adding this configuration using config file (default within current directory or home directory, but can be explicitly ordered to read config from given path) or command line options would be appreciated!

[mtshiba]

A command option already exists to specify the interpreter to be used.

https://github.com/erg-lang/erg/blob/main/doc/EN/command_options.md#--py-command---python-command

The configuration file is currently under specification design. Please wait for a while.

Also, currently, Erg's bytecode backend only supports CPython. If you want to use other interpreters, please use the Python script backend (using transpile subcommand), which is an experimental feature.