Feature proposal: Using transformers in plaintext

[brc-dd]

Clear and concise description of the problem

Currently, the comments based solution doesn't work with plaintext blocks:

```txt
foo // [!code --]
bar // [!code ++]
```

This used to work with Enzo's shiki-processor implementation because it was using a regex-based solution to do stuff regardless of the language.

x-ref: vuejs/vitepress#4602

Suggested solution

We can support the syntax above or maybe without need for comment markers in plaintext? This workaround seems to work fine for simple cases:

codeTransformers: [
  {
    span(span, i, j, line) {
      if (this.options.lang !== 'txt') return
      const text = span.children[0]
      if (!text || text.type !== 'text') return
      if (text.value.endsWith('[!code ++]')) {
        text.value = text.value.slice(0, -10)
        this.addClassToHast(this.pre, 'has-diff')
        this.addClassToHast(line, ['diff', 'add'])
      } else if (text.value.endsWith('[!code --]')) {
        text.value = text.value.slice(0, -10)
        this.addClassToHast(this.pre, 'has-diff')
        this.addClassToHast(line, ['diff', 'remove'])
      }
    }
  }
]

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.

[bsdayo]

Faced a similar problem today. I'm rendering a Markdown codeblock with diffs, but due to the fact that Markdown doesn't have comments, it's impossible to show a diff line in a Markdown codeblock.