Port amalgamate bash script to Python

[willwray]

The current bash script tupl/tupl_impl/amalgamate.sh is series of sed commands.

The bash script doesn't run natively on Windows so Python will be more portable.
The meson build already depends on Python, although CMake will not.
Python will be made an optional dependency via an opt-in build flag.
The ability to build the amalgamated header is seen as a developer option.
Consumers of the amalgamated header are most likely to download it from a Release.