Set up subcommands for non-core tasks

[dhruvkb]

Problem

pls is currently one command that does the core job. We should structure it to accept subcommands to do non-core jobs like the following:

• check for updates

  This should only check if a newer version is available. There is no need to automatically update the application, we let the package manager handle this. To be fair, most package managers will automatically install the update too.

• show help

  This should show the help that is currently triggered by --help. Help is technically a separate action and, conceptually, should not be an option within the main command.

• open the docs

  We have expansive in-depth documentation at https://pls.cli.rs. This command should print this URL, in a manner that terminals recognise as links, and also support opening the web documentation in the user's default browser.

• check the config

  This command should verify the pls config file to check if there are syntax errors, and unrecognised or deprecated keys (suggesting fixes for those).

• print diagnostic info

  This command should print some diagnostic information such as the pls version, the OS name and version and other dependencies like Git and the current Terminal emulator. This makes it easier to reproduce bugs reported by users.

Description

Add pls subcommands to do the above mentioned things.