feat: Single settings file generation

[mkundu1]

A new settingsgen.py has been added to write the settings API classes in a single file. The old one is still available behind PYFLUENT_USE_OLD_SETTINGSGEN environment variable to triage any issue due to the new mechanism. The filesize of the generated file together with the stub file is about 5 MB in the new mechanism:

D:\ANSYSDev\PyFluentDev\pyfluent>ls -l src\ansys\fluent\core\generated\solver      
total 9692
-rw-r--r-- 1 mkundu Domain Users 2837400 Oct 11 14:07 settings_251.py
-rw-r--r-- 1 mkundu Domain Users 1904201 Oct 11 14:07 settings_251.pyi

This has reduced the .whl size from 36 MB (0.26.1 build) to 5.5 MB.

Testing:

1. Ran Fluent regressions
2. Compared api_tree with the old mechanism.

[hpohekar]

Looks good. Thank you.