Improve performance of SyntaxHighlightFilter

## What does this MR do?

This MR improves the performance of `Banzai::Filter::SyntaxHighlightFilter`. See e9bacc6575d0002c6cab620075dea3dc7f93f100 for more information.

## Are there points in the code the reviewer needs to double check?

Styling mostly.

## Why was this MR needed?

Syntax highlighting is rather slow.

## What are the relevant issue numbers?

#18592 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] ~~Added for this feature/bug~~
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5643