Support enablement for walkthrough steps

Please consider supporting `enablement` for walkthrough steps, similar to `viewsWelcome`.

For example, a walkthrough can have these steps:

1. Create project
2. Build project

The _Build project_ step should be visible from the beginning, so the user can see what comes next. However, its commands should be disabled until a project is created. (This is just a made-up example.)

If a step contains multiple commands (inline links or action buttons) and its `enablement` context is false, disable all commands.

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Currently, one can use `when`, but that hides the content. As a workaround, it's possible to create two conditional steps: one without the commands and another with the commands. This also means the content changes when the condition changes.

The same problem was discussed when `enablement` was added to VS Code `viewsWelcome`. The reasoning was:

> can have 100% the same contents thus causing no jumping at all

Ref:
https://github.com/microsoft/vscode/pull/107705#issuecomment-701315100

Please consider aligning `walkthroughs` with `viewsWelcome` by supporting both `when` for visibility and `enablement` for command availability.

Thank you!