Add support for --check flag in update command

[Midnightific]

Closes: #47

Implements support for a --check flag in the update command to check if there are any updates for installed tools but will not update the tools in question

Test used:

1. Make a random folder
2. Run rokit init
3. Change version of package(s) to an outdated version
   3.1 Used Rojo version 7.4.1 to check for updates (expected to resolve version 7.4.2)
   3.2 Used wally version 0.3.1 to check for updates (expected to resolve version 0.3.2)

Test resolved both of the correct up to date versions :)

I'm new to rust so any feedback is appreciated! 🙃

[filiptibell]

Thank you for the PR! I'd like to see two changes before merging this:

• The new pub check: bool needs a code comment above it so that documentation shows up when running rokit update --help
• We should probably do an early return instead of if self.check { /* print versions logic */ } else { /* update versions logic */ }, it will make the diff cleaner and keeps the spirit of the new arg a bit better (--check = exit early, exit after check)

Everything else looks good!

[filiptibell]

One last thing - it seems like a comment got removed accidentally.

// FUTURE: Install the newly updated tools automatically

We should keep this since the update command does not do this yet.