Add devcontainer/Codespace support

[dinhtungdu]

This PR is a refactor of #90 with notable changes:

• Use the official WordPress image instead of the 10up's one.
• Use simpler and cleaner .devcontainer folder.
• Remove env variable hack.
• Has Xdebug configured and working.
• Support M1 devices.

Testing

devcontainer

1. Install the Remote Development extension pack.
2. Make sure Docker is running.
3. Open this repo in VS Code, and check out this PR.
4. From Command Palette, select `Reopen In Container'.
5. See the editor setting up the environment.
6. After every is setup, see VS code open /var/www/html folder.
7. Visit http://localhost:8080. See the development site is installed.
8. Log in to the admin with admin and password for username and password respectively.
9. See the Simple Podcasting is built and activated.