Add devcontainer configuration for GitHub Codespaces.

[jedieaston]

This commit adds a devcontainer.json file and some associated configuration files to automatically configure a Canvas development environment in GitHub Codespaces (or locally, via the VSCode remote containers extension). This way, all you have to do is click "New Codespace" (or open the repo in VSCode locally), wait ten minutes, and be ready to contribute!

Test Plan:

• Create a new Codespace using a branch with these files. (Or open it in VSCode locally, and follow the prompts to build a devcontainer.)
• Make sure it builds successfully.
• Run "bundle exec rails db:initial_setup" and "bundle exec rails server" to make sure Canvas comes up successfully.

[CLAassistant]

All committers have signed the CLA.