llama : add --completion-bash option (#11846)

This commit adds a new option `--completion-bash` to the llama.cpp which
outputs a source-able bash completion script.

The motivation for this change is to provide a more user-friendly
experience for users who use the command-line interface of llama.cpp.

This is currently only basic and all options are displayed for all llama
executables but this can be improved in the future if needed.

Example usage:
```console
$ build/bin/llama-cli --completion-bash > ~/.llama-completion.bash
$ source ~/.llama-completion.bash

$ ./build/bin/llama-server --m<TAB>
--main-gpu         --mirostat         --mirostat-lr      --model            --multiline-input
--min-p            --mirostat-ent     --mlock            --model-url
```

README.md

@@ -521,3 +521,17 @@ If your issue is with model generation quality, then please at least scan the fo
 
 #### References
 
+
+### Completions
+Command-line completion is available for some environments.
+
+#### Bash Completion
+```bash
+$ build/bin/llama-cli --completion-bash > ~/.llama-completion.bash
+$ source ~/.llama-completion.bash
+```
+Optionally this can be added to your `.bashrc` or `.bash_profile` to load it
+automatically. For example:
+```console
+$ echo "source ~/.llama-completion.bash" >> ~/.bashrc
+```