Add ADD_NAMESPACES option to support strict XML documents

[kevin-deyoungster]

This pull request implements feature #728: Support additional namespaces in XML documents

Background & Context

DOMPurify cannot sanitize XML documents with custom namespaces. This PR introduces a CONFIG option ADD_NAMESPACES to enable users to allowlist custom namespaces to consider during sanitization.

Sample Usage

DOMPurify.sanitize(dirty, {
    PARSER_MEDIA_TYPE: "application/xhtml+xml",
    ADD_TAGS: [],
    ADD_ATTR: [],
    ADD_NAMESPACES: []
})

Tasks

• Add ADD_NAMESPACES config parameter
• Confirm from reviewers if feature can be supported
• Refactor
• Add to test suite: "Config-Flag tests: ADD_NAMESPACES"

[cure53]

Hey there, thanks for the PR, this looks generally good and I think supporting the feature as proposed makes sense!

[kevin-deyoungster]

@cure53, could you kindly review, thanks!

[kevin-deyoungster]

@cure53, awesome.

1. Could you add me as contrib. so I can merge the PR in?
   I don't have write permissions to this repo to trigger the merge.

2. Would I need to update type definitions in https://github.com/DefinitelyTyped/DefinitelyTyped/blob/15ecc0968b3e6eaa0bc9cd2377afa547a7f98653/types/dompurify/index.d.ts as well?

[cure53]

@kevin-deyoungster Awesome, thanks :)

I just did the merge and yes, it's likely necessary to send a small PR to the DT folks, thanks a lot!