`terminal.allowCommands` should have more strict default value

[AriPerkkio]

Is your feature request related to a problem?

By default the terminal allows users to use any available command. In tutorials this is usually not intended experience. The lesson authors should define commands that are allowed - if any.

Describe the solution you'd like.

Let's change the default value of terminal.allowCommands from [] to something like ['ls', 'echo'].

Lesson authors should still be able to allow all possible commands by specifying terminal.allowCommands: [] in metadata.

Describe alternatives you've considered.

Recommend authors to always specify terminal.allowCommands.

Additional context

No response