feat(cheatcodes): add docs for exists, isFile and isDir cheats

[ruvaag]

Add docs for new filesystem cheatcodes exists, isFile and isDir from foundry-rs/foundry#5719

• Update src/cheatcodes/fs.md with cheatcode sigs
• Update src/cheatcodes/README.md with cheatcode sigs

[Evalir]

Sweet, this so far lgtm. We also need a page for each cheatcode explaining what they do like https://github.com/foundry-rs/book/blob/master/src/cheatcodes/chain-id.md

[ruvaag]

Got it, thanks. Looks like the filesystem cheatcodes are grouped under src/cheatcodes/fs.md for now.

Would it be better to append an example block for exists, isFile and isDir on the same page to keep them grouped? If not, I can create separate pages for the three of these instead.

[Evalir]

gotcha! yep, let's group them up on fs.md