structured CLI apps

Stream: mathlib4

[Arthur Paulino]

There is a script folder with bash, python and lean files and the runLinter.lean file is there. However, runLinter is compiled to a binary when we call lake build. So I had the idea of structuring such sources in a CliApps folder and then making separate lean_exe configs for them, similar to what we have for Cache.

Cache, then, would be placed there as well. And runLinter.lean would be renamed to Linter.lean, whose binary call would be lake exe linter.

Another advantage is that the linter wouldn't be compiled for no reason when users call lake build.

This is not a radical change, but would allow a more organic way of mathlib4 having its own CLI apps. What do you think?