Add Free mode with basic ui

[morteako]

Adds basic free mode support.
Just an MVP, but should be fine
Treats 0 as free mode, and then shows this in the UI.
Adds a helping text in the editor to help peopple

For later:

• Have a better free mode option in the workout editor
• Have free mode with watt target in the workout editor

[sivertschou]

LGTMADTSC

[sivertschou]

Can call it free mode for now, but I can't really see Free mode_ being used as a term. Some mention Resistance mode, Standard mode and SIM mode – we should decide what to call this 🤠

Anyways, I have some minor suggestions for the text itself, and I also suggest moving the text beneath the Add part button.

      <Text fontSize="small">
        Tip: Use 0 as power target for <em>Free mode</em>
      </Text>

aka:

• Use fontSize instead of fontWeight
• We don't need {} around component String params
• Add spacing after Tip:
• Call it Power target instead of Watt target, since there is a column named Power above said field
• Free mode instead of Free Mode – That's also what's been used the other places it is mentioned in this PR 🤓
• Add emphasis around Free mode