Add cargo stylus script subcommand

[reo101]

Description

Context

Introduce a new cargo stylus subcommand script that enables executing Stylus program scripts that interact with the network.

Changes

• add script subcommand with 2 subcommands:
  • script run ${NAME} to run a script at ./scripts/${NAME}
    Currently runs make, i.e. scripts can be semi-trivially extended to support multiple languages, given one can write a Makefile with a "run" target
  • script new ${NAME} to create a blank script at ./scripts/${NAME}
    Includes some small utilities to aid the creation and application/realisation of templates
    Can be further extended to create more complete scripts, similar to the one in stylus-hello-world
• add two utility functions related to finding the Stylus project root
• cargo-stylus was previously a binary crate, now is a library + binary, where the binary just wraps the main_impl function from the library
  • expose all functions as a library (to be used in scripts, see NOTE)
• modify export-abi --json to filter the non-JSON parts of the output (deemed to be the first 3 lines, see NOTE in code)

Checklist

• I have documented these changes where necessary.
• I have read the DCO and ensured that these changes comply.
• I assign this work under its open source licensing.