fix: Hide update button if self-update is not available

[joehorsnell]

If you have Mise installed using a package manager, such as Homebrew, the plugin still prompts to install available updates, even though it will fail.

Ideally there would be data available in mise doctor, or some other command, to determine if we can self-update, but there is not, so this PR reimplements the is_available() function from SelfUpdate.

Fixes #122.

[joehorsnell]

Hi @hverlin - thanks for your work on the Mise VSCode plugin 👍.

I currently can't use the self-update feature of Mise at work because of jdx/mise#5459, so I'm just installing it via Homebrew, which nags me on each new version, so I thought I'd submit a fix for this issue.

There don't appear to be any tests for MiseService, but happy to add some if you suggest how? That class seems to be quite closely coupled to the VSCode extension API, as it takes a vscode.ExtensionContext in the constructor. I'm not too familiar with those APIs, so I'm not sure how to unit test those functions without getting into a mock-fest.

[hverlin]

Thank you for contributing! Ideally we should indeed rely on mise doctor output but this will do until it's eventually added to mise itself.

[joehorsnell]

Thank you for contributing! Ideally we should indeed rely on mise doctor output but this will do until it's eventually added to mise itself.

That was quick! Thanks @hverlin 👍

Will you cut a new release with this change in it?

[hverlin]

A new release should be available now ✅