Add important to custom containers

[JesusTheHun]

Is your feature request related to a problem? Please describe.

GitHub-flavored Alerts features 5 colors while the custom containers only feature 4. I would like to avoid mixing the syntaxes to create colored containers.

Describe the solution you'd like

Add important custom containers that feature the same color as their github counterpart.

Describe alternatives you've considered

Mixing syntaxes.
Using the tip custom container.

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the docs.
• Read the Contributing Guidelines.
• Check that there isn't already an issue that asks for the same feature to avoid creating a duplicate.