fix(plugins/links): mark link_open as reflink

[plugins/links](https://github.com/diplodoc-platform/transform/blob/master/src/transform/plugins/links/index.ts)

We have reflink syntax, this markup `[{#T}](file.md)` extracts title from `file.md` and uses it as link text.

We need to mark `link_open` token with `markup: reflink`(analogous to how `autolinks` are parsed).

Right now i am doing it inside [@diplodoc-platform/markdown-translation](https://github.com/diplodoc-platform/markdown-translation) library when rendering tokens into XML.

Objectively links of reflink type should just be marked as such by parser.

to easier understand the token structure checkout tokens tab: [link to the playground](https://diplodoc-platform.github.io/playground/?input=%255B%257B%23T%257D%255D%28file.md%2520%2522title%2522%29)