Move from sys.argv to argparse

[FelixWolf]

Hoping I understood the "No API changes" policy in the contributing correctly here. (Interpretted it as "No changes to the LSL api", rather than "No changes to the script")

Currently the script is using sys.argv for parsing arguments, which works, with some caveats.
If a argument is missing, it raises a IndexError, rather than an error message on what type of argument is missing.
In addition, there isn't a -h / --help to help with listing arguments and available parameters. While this is obtainable by reading the source code, I believe that it would be more helpful to have self documenting arguments.

Before:

[felix@crocuta lsl-definitions]$ python3 gen_definitions.py
Traceback (most recent call last):
  File "/home/felix/source/other/lsl-definitions/gen_definitions.py", line 1968, in <module>
    main()
    ~~~~^^
  File "/home/felix/source/other/lsl-definitions/gen_definitions.py", line 1931, in main
    definitions = parser.parse_file(sys.argv[1])
                                    ~~~~~~~~^^^
IndexError: list index out of range

After

[felix@crocuta lsl-definitions]$ python3 gen_definitions.py
usage: gen_definitions.py [-h]
                          definitions
                          {syntax,gen_constant_lsl_script,gen_lexer_file,gen_parser_file,gen_tree_header_file,gen_tree_source_file,gen_cpp_constants,gen_func_call_scripts,gen_lscript_library_defs,gen_mono_library_defs,gen_lscript_interface,gen_mono_bind_interfaces,gen_lscript_library_bind_pure,gen_builtins_txt,gen_lua_registrations} ...
gen_definitions.py: error: the following arguments are required: definitions, mode


[felix@crocuta lsl-definitions]$ python3 gen_definitions.py lsl_definitions.yaml gen_lexer_file --help
usage: gen_definitions.py definitions gen_lexer_file [-h] template_file output_file

positional arguments:
  template_file
  output_file

options:
  -h, --help     show this help message and exit

This change was done in a manner to ensure that the arguments follow the same exact syntax as before, as such existing scripts will not break.
There is room for improvement if existing scripts could be modified, for example with gen_lua_registrations, instead of specifying 0 or 1, we could do --pure as a optional argument, or using argparse's FileType to allow output to stdin / stdout. However a change like that I feel needs to be further looked into and approved, to not break internal scripts that rely on this.
I feel, for now, the current changes are sufficient enough to help people further test yaml changes and improvements they make.

[HaroldCindy]

Looks reasonable to me, thanks!

[HaroldCindy]

@FelixWolf Looks like it might require some formatting changes to pass CI https://github.com/secondlife/lsl-definitions/actions/runs/16227903266/job/45978129373?pr=6