Fix missing executable extensions in tool links

[filiptibell]

This PR fixes #13 by properly including the OS executable extension when creating tool links.

Also included in this PR:

• Automatically fixing existing invalid links (ones missing an executable extension) when running rokit self-install and / or rokit install
• A loud, user-facing warning message for invalid executable files being written, to prevent this from happening in the future, or at least make it easier to diagnose
• Switches to a single "source of truth" internally for discovering existing tool links

[kennethloeffler]

I can't check that recreating the tool links during rokit self-install works because something else is going wrong when I run rokit self-install over an existing installation:

But everything else (besides the loud warning) is working well:

[kennethloeffler]

LGTM, I'll investigate why using self-install over my existing installation didn't work and open an issue if needed later

(new release? 👀)

[filiptibell]

LGTM, I'll investigate why using self-install over my existing installation didn't work and open an issue if needed later

(new release? 👀)

Sounds good to me! Will update the changelog and run a new release 🫡