Switch the syntax highlighting to Prismjs

Fixed: https://github.com/runtimeverification/web-common/issues/87

Please make sure https://github.com/runtimeverification/iele-semantics/pull/206 is merged before this PR gets reviewed and merged.

Demo: https://shd101wyy.github.io/iele-semantics/

This PR switched the syntax highlighter library from highlight.js to prismjs.

This PR added syntax highlighting for the following languages in the rendered HTML pages from Markdown.

[x] K
[x] IELE

This PR also fixed some dark mode issue.

You could simply highlight iele in markdown with the following notation:

```iele
contract ERC20 {

  // constant used for shifts
  @addressShift = 160
  ...



and it will show the result like below:

![Screenshot from 2020-11-16 19-04-24](https://user-images.githubusercontent.com/1908863/99245560-baae4a80-283e-11eb-8704-d4a79dfba523.png)

runtimeverification / iele-semantics

Switch the syntax highlighting to Prismjs #209