Support custom heading classes in addition to IDs

[mering]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Description

In addition to custom heading IDs, also allow setting custom heading classes.

Has this been requested on Canny?

No response

Motivation

This allows custom styling via CSS or filtering via JS.

API design

For example, I can imagine a syntax as follows:

# Heading {#id .class1 .class2}

Have you tried building it?

This could be implemented in the heading remark plugin where custom IDs are already processed.

Self-service

• I'd be willing to contribute this feature to Docusaurus myself.