Change extension folder structure to reduce file name conflicts

[rh101]

Which branch your pull-request should merge into?

• 2.1: BugFixes and Improvements for Current LTS branch

Describe your changes

Moved each extension into its own "src" folder.
Each extension CMakeList.txt now sets up the include paths for that extension.
LUA bindings generator updated, but this needs to be checked.
Fix issue with detecting Effekseer and DrawNodeEx extensions in cpp-tests.

Note that I manually updated the auto-generated lua bindings, so I haven't yet tested the changes to the LUA binding generator to see if it does generate the correct output. Hopefully that is the case, but if not, then any help with that specific part would be appreciated.

It seems that a few existing issues surfaced when the extension folder structure was changed, such as header files to inactive extensions being included when they shouldn't be.

FYI, the changes in this PR shouldn't break any existing code. If anything, it would be a few include statements that would need to be fixed up in a project, assuming it uses extensions.

Issue ticket number and link

#1645

Checklist before requesting a review

For each PR

• Add Copyright if it missed:
  - "Copyright (c) 2019-present Axmol Engine contributors (see AUTHORS.md)."

• I have performed a self-review of my code.

  Optional:

  • I have checked readme and add important infos to this PR.
  • I have added/adapted some tests too.

For core/new feature PR

• I have checked readme and add important infos to this PR.
• I have added thorough tests.

[rh101]

There are issues with the LUA bindings generator, so attempting to fix that in order to complete this PR.

[rh101]

The LUA binding generator should be working now. If there are any concerns or suggestions, then I'll get around to having a look at them in 2 days time.

[halx99]

lgtm

[rh101]

Hold off on any merging. One last thing to do, which is fixing the pre-built paths in AXLinkHelpers.cmake.

EDIT: All done. Commit d894c54 should also fix the issue with yasio that has been reported on the Discord server (missing header search path and linker error with pre-builts).

[halx99]

@rh101 the bot try override luabindings: #1648