samples: llext: shell_loader: add hello_world.c, update README

Hello world from the tests is no longer what it once was, a simple
hello_world function that prints. The sample was here to let people
explore and learn about how LLEXT and ELFs work.

Moving the sample extension code to the shell loader sample disconnects
the two and lets the sample remain simpler.

We use the new CONFIG_LLEXT_IMPORT_ALL_GLOBALS to show how to build
an extension with and without the Zephyr build system.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>