Adds a bash completion script for the new bitcoin command-line
tool, which unifies the main Bitcoin Core executables under a
single interface. This feature improves usability, reduces errors,
and makes the command-line tools more easily discoverable for users
working in a Linux bash environment.
The completion script dynamically lists available commands and options
by parsing `bitcoin --help` and `bitcoin help`. It also incorporates
the existing bash completions for `bitcoind`, `bitcoin-cli`, and
`bitcoin-tx`, depending on the argument provided (node, rpc, or tx).
This ensures that all relevant completions are available seamlessly
through a single interface without modifying core functionality.
No functional changes to core code are introduced; this is an optional
enhancement placed under `contrib/` for easy installation and use.
This naming scheme supports auto-detection and on-demand loading of completions.
See
ba109693ee/README.md (faq),
section "Where should I put it to be sure that interactive bash shells will find it and source it".
Previously, distro package maintainers had to rename these files manually.
