Feature proposal: Support registering lang alias after creation

[fuma-nama]

Clear and concise description of the problem

While it is possible to load languages after instance creation, there's no API for registering lang alias after creation. It is quite useful when I want a global Shiki instance while the aliases are load dynamically.

Suggested solution

I think there should be a highlighter.setLangAlias(alias, lang) & highlighter.unsetLangAlias(alias) API.

I'm willing to open a PR if approved.

Alternative

No response

Additional context

No response

Validations

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