Syntax highlighting automatic color scheme switching (#815)

### Prerequisites

Put an `x` into the box(es) that apply:

- [ ] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

This PR accomplishes the following:

- Adds GitHub light and dark syntax highlighting styles to the theme as
baseline (since it is already the 'default' style).
- Automatic syntax highlighting color scheme switching according to the
light and dark theme toggle.
- Syntax highlighting is by default no longer broken for users with a
defined Content Security Policy (CSP) since the CSS files are generated
and included with the theme natively rather than being loaded
externally.
- Allows users to set or override with different styles if needed by
removing `noClasses = false` (which defaults to true) and defining a
`style` under `markup.highlight` in `config.toml`. Alternatively, those
who want to use custom styles (or have a CSP) can simply and easily
override `_syntax.scss` and `_syntax_dark.scss` which now already exist
as a base with their own CSS styles, which also works better with the
theme and supports automatic color scheme switching.

### Issues Resolved

This is a very sane implementation of #331 with sensible defaults and a
good base experience for users who have not explicitly defined a `style`
for `markup.highlight`.

### Checklist

Put an `x` into the box(es) that apply:

#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [x] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [x] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already