let walkTokens replace the current token with multiple

Describe the feature
Currently, walkTokens lets you edit individual token properties as you walk the tree. However, you can't inject new tokens (except as children if the current token has a "tokens" property). I would like to take, say, an HTML token, and break it apart into multiple pieces as their own tokens. For example:

<div>content</div> is parsed into:

[
{type:"html", block:true, raw:"<div>\ncontent\n</div>", pre:false, text:"<div>\ncontent\n</div>"}
]

Which I would like to split into:

[
  {type:"html", block:true, raw:"<div>\n\n", pre:false, text:"<div>\n\n"}
  {type:"text", raw:"content", text:"content", escaped:false}
  {type:"html", block:true, raw:"</div>", pre:false, text:"</div>"}
]

Why is this feature necessary?
walkTokens is a great tool to tweak the token tree, particularly when you want to slightly change the behavior of a built-in token without reimplementing the entire tokenizer along with its regex. But it would be nice to have more control over the tree beyond just changing the existing tokens.

Describe alternatives you've considered
Fully re-implementing the tokenizer for built-in tokens does work, because you can break tokens down as much as you want, but extensions can quickly fall out-of-sync with Marked.js bug fixes, and requires a lot more maintenance work.

Start by tracing the existing walkTokens traversal and the tokenizer behavior for HTML tokens. Determine how a current token could be replaced by multiple tokens while preserving tree traversal, then verify that the described HTML example produces the three-token result without breaking existing token edits.