Feature Request: add a way to register dependencies by plugins

[sapphi-red]

Plugins may rely on a different file but there's no way to tell that information to lightningcss.

For example, the plugin below depends on .json placed next to the input file.

export function fooVisitor() {
  /** @type {string[]} */
  let currentStyleSheetSources
  return {
    StyleSheet(stylesheet) {
      currentStyleSheetSources = stylesheet.sources
    },
    Rule: {
      unknown: {
        foo(rule) {
          const from = currentStyleSheetSources[rule.loc.source_index]
          const importedFile = JSON.parse(fs.readFileSync(from + '.json', 'utf8'))
          const cssAst = convertJsonToAst(importedFile) // convert json to lightningcss somehow
          return cssAst
        },
      },
    },
  }
}

To make the re-transform happen when the json file is changed, the user that calls lightningcss.bundle needs to know which css file depends on which json file.

Simply having a convention (e.g. expose dependencies property from the visitor) would work if composeVisitors supports it, but having the information in TransformResult.dependency feels natural.

For reference postcss, achieves this via messages.
https://github.com/postcss/postcss/blob/main/docs/guidelines/plugin.md#31-use-messages-to-specify-dependencies
Maybe it makes sense to have a way to emit warnings from visitors as well.