Move entry points into their folders

[fregante]

• Follows Rename devTools to pageEditor across the extension #2853

As previously suggested by Todd, this PR moves the entry points into their folders. I had initially thought about naming them background/main.ts but Todd suggested it's annoying to have many files with the same name laying around, so:

background.ts -> background/background.ts

For:

• contentScript
• background
• options
• sidebar

I moved small contexts that don't have their own sub-folders into a joint tinyPages folder. This only serves to clean up the src root and indeed it's looking quite clearer now.

• tinyPages/ephemeralForm.tsx
• tinyPages/permissionsPopup
• tinyPages/frame.ts
• tinyPages/devTools.ts (not to be confused with pageEditor)

I also merged the 2 global .d.ts files and moved the headers.ts to the development folder.