Stop auto detection of syntax highlight language

[yhatt]

By the historical background, Marp has detected the language for syntax highlight in the code block automatically until now.

```
const a = 1
```

However, this behavior is a bit confusable because the other services using Markdown have not colored the plain code block. e.g. marp-team/marp-vscode#12 had mentioned to this problem.

In addition, recently we've recieved the potential ReDOS vulnerabilities in GHSA-7wwv-vh3v-89cq (Fix in #201). Using highlightAuto may be vulnerable.

marp-core/src/marp.ts

Line 98 in daa5060

return highlightjs.highlightAuto(code).value

Thus, we have to consider stopping auto detection of syntax highlight language.

This change will break existed slides, and affected user should specify the language manually. But I think this is a natural behavior as Markdown.