path fs: move `path.exists*` to `fs.exists*`

path fs: move `path.exists*` to `fs.exists*`

`path.exists*` functions show a deprecation warning and call functions
from `fs`. They should be removed later.

test: fix references to `path.exists*` in tests

test fs: add test for `fs.exists` and `fs.existsSync`

doc: reflect moving `path.exists*` to `fs`