chore: support specify contract path input with or without -p flag

[AlexD10S]

Currently, contract commands use the -p flag to specify the path to a contract or artifact file. The experience can be improved if pop-cli supports both options: using the -p flag and specifying the path directly without it (subcommand).

Example:

pop call contract -p ../contract.json 

can simply be

pop call contract ../contract.json

This improvement should be consistent across all contract-related commands, including:

• pop build
• pop up contract
• pop call contract

[ndkazu]

Hello, I started working on this issue in PR #361 and confirmed that the improvement is working as expected for pop build.
pop call contract & some additional tests are still needed: Work in progress.

[ndkazu]

@AlexD10S , I made a cleaner version of my first suggestion (modifications are minimal, and limited to pop build ...). I am still looking for other (less invasive?) routes, but have a look at my additional comments in the PR, and let me know if it makes sense to propagate this method to pop up contract & pop call contract.