[WIP] feature/addon updater

[UuuNyaa]

DO NOT MERGE YET

I suggest the following improvements.

1. Add .gitignore file
2. Release Workflow
3. Change the structure of release artifacts
4. Add-on updater feature

I would like to discuss which improvements would be acceptable.

[UuuNyaa]

1. Add .gitignore file

Please check the "Files changed"

2. Release Workflow

Add a GitHub Action that will automatically create a release draft when a tag is pushed.

3. Change the structure of release artifacts

This only applies to release artifacts. The source zip will not change.

Actual

mmd_tools/
    panels/
    properties/
    core/
        ...
    operators/
samples/
    pmd/
    blends/
    pmx/
tests/
    output/

Expected

mmd_tools/
    panels/
    properties/
    core/
        ...
    operators/

4. Add-on updater feature

Add add-on update feature and UI to preferences.

[powroupi]

Sorry, I'm busy on my job, so I'm afraid not being able to maintain this project frequently, I may stop maintaining it from now on.:cry: If you are interested in maintaining this project, you can keep updating it on your fork and redirect users to your fork. Thank you for all users' and contributors' support. :smile:

[UuuNyaa]

@powroupi Thank you for taking the time to reply to me. I will positively consider taking over the maintenance. I'll mention to you when I'm ready to take over.