Add support for Mermaid.js diagrams

[dessalines]

Discussed in https://github.com/LemmyNet/lemmy-ui/discussions/2735

There is a markdown-it plugin for mermaid: https://github.com/wekan/markdown-it-mermaid , that I'll add. Any bugs related to it tho, you'll have to open up over there.

cc @YousufSSyed

[dessalines]

Nevermind, it doesn't work because that library doesn't support ESM.

Error [ERR_REQUIRE_ESM]: require() of ES Module /home/xxx/git/lemmy-ui/node_modules/.pnpm/[email protected]/node_modules/mermaid/dist/mermaid.core.mjs not supported.

[Nutomic]

PR was closed as its too much bloat.