Support "lang" attr for i18n tag

[antfu]

I am working on a vscode extension for vue-i18n called vue-i18n-ally and trying to add support for SFC.

The current approach for YAML support in the document is not ideal for static analyze. Would it be possible to add support for lang attr just like script in SFC (which can also support more preprocessors)?

For example:

<i18n lang='yaml'>
en:
  hello: Hello World
</i18n>

<i18n lang='json5'>
{
  // comments
  ja: {
    hello: 'こんにちは、世界！',
  }
}
</i18n>

This issue is relative to #10, #31 and conflicted with #11

[antfu]

@kazupon Thanks for such a quick and awesome work!