feat(themer): add code highlight theme configuration and tests

[m-xim]

Description

Added support for configurable code highlighting themes.

Now documentation can use different highlight.js themes for light and dark UI modes through .yfm:

codeHighlight:
  light: github
  dark: github-dark

theme.yaml

Existing theme.yaml color overrides continue to work with the selected code highlight themes.

For example:

light:
  code: '#111111'
  code-background: '#ffeeaa'

dark:
  code: '#eeeeee'
  code-background: '#221144'

These values are applied on top of the selected highlight.js theme for .hljs, so users can select a full code theme and still override supported Diplodoc code colors.