Feat: unsaved file content

[tofarr]

Before Changes

When you make changes in the code editor, if you click into a different file, the changes are lost.
e.g.:
I write some changes:

But if I click into a different file, they are lost! (There is no dialog or warning to prevent this!)

After Changes

We store changes even after you click out of a file. Files with unsaved changes are marked with a "*", and a cancel button was added to clear changes. (My original idea was to use local storage / indexed db to store changes, but I have not implemented this for this iteration - so a page refresh will still clear edits)

I can click to a different file and make changes there too:

Clicking back to the original displays the unsaved changes:

Changes are only saved / cancelled when the page is refreshed or the button is clicked.

Under the hood

I added an array of fileStates to the redux state - this allows for determining whether there is unsavedContent, and displaying different UI states in the file explorer

Open Questions

I think some UI review is in order - making sure this is consistent with where we want the product to go, and that this UI is intuitive.

[amanape]

Sorry for the delay, I'll give it a full review tomorrow! LGTM though from a quick skim if you're in a rush

[amanape]

LGTM! Consider extending the test cases to serve as another form of documentation for newcomers (if you have the time of course!)

[tofarr]

Added a few more minimal test cases. TBH I think we need more test cases around editing