Shell: Add where builtin

The builtin is based on the behaviour of the z-shell.
Namely it tries to resolve every argument one by one.

When resolving (in the order below) the following results can occur:
  1. The argument is a shell built-in command. Then print it.
  2. The argument is an alias. In this case we print the mapped value.
  3. The argument was found in the `PATH` environment variable.
     In this case we print the resolved absolute path
     and try to find more occurences in the `PATH` environment variable.
  4. None of the above. If no earlier argument got resolved,
     we print the error `{argument} not found`.

If at least one argument got resolved we exit with exit code 0,
otherwise 1.

By not using Core::File to resolve the executable in the environment
but rather using a modified version of the code we print every
matching executable of the given name. This behaviour matches
up with the z-shell.

The builtin has the following flags to modify the behaviour according
to the users needs:
  - `-p --path-only`: This skips the built-in and alias checks
  (step 1 & 2)
  - `-s --follow-symlink`: This follows the symlinks of an executable to
  its symlink-free location.
  - `-w --type`: This displays the type of the found object
  without any additional descriptive information.

Userland/Shell/Shell.h

@@ -25,6 +25,7 @@
 
 #define ENUMERATE_SHELL_BUILTINS()     \
     __ENUMERATE_SHELL_BUILTIN(alias)   \
+    __ENUMERATE_SHELL_BUILTIN(where)   \
     __ENUMERATE_SHELL_BUILTIN(cd)      \
     __ENUMERATE_SHELL_BUILTIN(cdh)     \
     __ENUMERATE_SHELL_BUILTIN(pwd)     \