add meta to markdown-it env

[kagawagao]

Is your feature request related to a problem? Please describe.

there is no context info currently, such as filepath/vite compiler, when markdown-it parse markdown files, and this leads to the inability to resolve relative file path when parsing markdown files, or add dependency to compiler to make hmr work as expected

for example, snippet

https://github.com/vuejs/vitepress/blob/ea6cb00c3e9099d45fc236bff0f7c42a65d558dc/src/node/markdown/plugins/snippet.ts?_pjax=%23js-repo-pjax-container%2C%20div%5Bitemtype%3D%22http%3A%2F%2Fschema.org%2FSoftwareSourceCode%22%5D%20main%2C%20%5Bdata-pjax-container%5D#L145-L147

// loader always undefined
if (loader) {
  loader.addDependency(src)
}

Describe the solution you'd like

add meta to markdown-it be called) when render

Describe alternatives you've considered

No response

Additional context

No response

Validations

• [X] Follow our Code of Conduct
• [X] Read the docs.
• [X] Read the Contributing Guidelines.
• [X] Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.

[brc-dd]

#869 will add file path in env. Will track that loader thing in #117.