various version manager improvements

[Techatrix]

This PR applies three changes to the version manager. The changes correspond the first three commits.

offer to update workspace config files when selecting Zig version

fixes #247

Using the "Select and Install" popup to switch to a different Zig version will no longer be saved permanently. It will revert to the previous version after restarting VS Code. To make the change permanent, the version should be saved in a .zigversion, build.zig.zon or settings.json file. The extension will automatically suggest this change with a message. If not workspace config has been found, a .zigversion file will be created.

I am open to any better wording suggestions for the message. This is what I went for:

lookup Zig in the $PATH if no workspace version has been specified.

If the workspace specified no Zig version, the extension would automatically install the latest tagged release. This commit will try to lookup Zig in the $PATH before falling back to the latest tagged release. This is meant to avoid confusing users why their already installed Zig version is not being used.

override the $PATH in the integrated terminal

Previously, if the $PATH has a zig version, the integrated terminal would prefer it over the version that is used by the version manager. Thanks to the previous commit, it now makes more sense to override the $PATH. This means that the integrated terminal will use the Zig version requested by the workspace even if the user added Zig to the $PATH.

[Vexu]

LGTM!

I don't have a better idea for the message but asking to "save Zig version in this workspace" when not in a workspace seems strange. Maybe it should have a more specific message or just save the version in settings without asking since workspaces can override it.