Codeblocks syntax highlighting and background

[ErikMichelson]

Description

Codeblocks (with fences) do not have a background-color. Thus it's harder to distinguish them from other text content.
Another effect hardening that problem is the fact that for certain languages the syntax-highlighting keywords do not match the autocomplete suggestion.

Steps to reproduce

Create a new note with the following content (ignore the backslashes) and view it both in light and dark mode in HD2. Compare it also with the results in HD1:

\```
test codeblock without background
\```

Invalid autocomplete suggestions:
\```ts
// ts vs typescript as language
const a = {
  "a": -5
}: Record<string, number>;
const b = window.atob(JSON.stringify(a));
window.open(`https://hedgedoc.org/#x=${b}`)
\```

\```js=
// neither js nor javascript is suggested
const a = {
  "a": -5
};
const b = window.atob(JSON.stringify(a));
window.open(`https://hedgedoc.org/#x=${b}`)
\```

Expected behaviour

Autocomplete suggestions should (only) include the languages that are supported by our rendering syntax highlighting.
Codeblocks should have a decent background color to distinguish them from the other content.

Logs

No response

Config

No response

Your Setup

• hedgedoc.dev
• Chrome 123, Ubuntu 23.10

Additional context

vs