This commit moves the import of mermaid into a `require.ensure` block,
that is only executed when a mermaid diagram is actually present
in a note. Webpack automatically splits the library into a separate
chunk and loads that on demand.
To ensure that mermaid code-blocks are not treated as normal
code-blocks while the chunk is loading, a corresponding check is added
to `finishView`.
Signed-off-by: David Mehren <git@herrmehren.de>
b45b8b9c0d