Bangle.js: Avoid storage compaction if battery low, show status

[gfwilliams]

Discussed here: https://github.com/espruino/BangleApps/issues/2865

Generally writes to storage are pretty safe and won't cause issues if the battery dies during a write, but if the storage is full then it has to be compacted. This can take a minute or more, during which a decent amount of power is used - and if the battery is very low then it could cause a shutdown during that process, which would cause storage corruption.

To make matters worse, if storage were very full and a file kept being changed, that could force storage be compacted multiple times.

So we need to:

• [x] Not start compaction if battery is below 10% - just error right away
• [x] Show compaction status onscreen (so the user doesn't think the Bangle has just crashed) - can be done with an Overlay on Bangle.js 2
• [x] See if we can be smarter about what we compact - if storage was compacted recently it may be the first 1MB+ of storage is already compacted and so we don't need to touch it

[gfwilliams]

We're still not being smart about where to start compaction...

jsfGetStorageStats could maybe store the address of the first page (that's not straddled by a file) with an erased file in it, and then I believe jsfCompactInternal should be fine to start from that

[gfwilliams]

Turns out it wasn't that bad :)