--withoutLanguageVariants should be available for the `start` step as well

[kelson42]

... and not only for the get step like written in the README.md.

[benoit74]

This is true for all settings.

This is more or less fixed with the new CLI phet2zim introduced in #231

More or less because npm run start still doesn't support flags.

But this is painful to fix and probably not worth it since we are not expected to use npm run start much now we have the proper CLI. And this is true for export step as well. This is more a design flaw (scripts do too much in one call) than an issue to fix from my PoV. I propose to postpone this.