Extract bash completion script as "external" resource

[albestro]

IMHO this helps in different ways:

• code readability, both on C++ and bash side
• tracking changes: there is no "workaround" or compromise due to nesting/raw string literals/... it will be easier to understand and apply changes
• development: being a real bash script, you can edit with your preferred editor (which might provide syntax highlighting, ...)
• "live"-patching: being a separate file, you can directly source it and use it during development without rebuilding, but actually using the real one

How this is achieved

• move the raw-string-literal content in a dedicated bash file

• generate with xxd a C header file at build-time containing the binary representation of the script;

• the custom target generating the header file is a new dependency of the uenv executable, so whenever the bash script changes, it gets regenerated and `uenv gets rebuilt

• include the build-time header file replacing the raw-string-literal

• use string_view to recreate the compile time string starting from the binary representation

• I should probably add xxd as a requirement