GitLab TOC support

Hello! Your extension is very convenient, but there are a few issues with the TOC, generated with markdown.extension.toc.githubCompatibility=true when using with GitLab (v. 11.11.3-ee, the latest one)

The first issue is the same I described here for doctoc. The others are specific for this extension (doctoc processes them correctly).

Here is my test.md with the generated TOC:

- [MikroTik hAP ac²](#mikrotik-hap-ac²)
  - [Switch + Access Point](#switch--access-point)
  - [Настраиваем Доступ к Сети](#Настраиваем-Доступ-к-Сети)

## MikroTik hAP ac²

The [hAP ac²](https://mikrotik.com/product/hap_ac2) is a Dual-concurrent Access Point, that provides Wifi coverage for 2.4 GHz and 5 GHz frequencies at the same time. Five 10/100/1000 Ethernet ports provide Gigabit connections for your wired devices, USB can be used for external storage or 4G/LTE modem, and device supports IPsec hardware acceleration.

### Switch + Access Point

### Настраиваем Доступ к Сети

1. The 1st entry contains a specific symbol ². I described a similar issue here. The real GitLab link for that header is

https://mydomain.tld/myrepo/test.md#mikrotik-hap-ac

1. The 2nd entry. Your extension incorrectly processes the + sign. The real GitLab link for that header is

https://mydomain.tld/myrepo/test.md#switch-access-point

1. The 3rd entry. This is a Cyrillic text. Your extension does not convert upper case characters to lower case. The correct TOC entry for that header (the working one) should be

  - [Настраиваем Доступ к Сети](#настраиваем-доступ-к-сети)

And the real GitLab link for that header is

https://mydomain.tld/myrepo/test.md#%D0%BD%D0%B0%D1%81%D1%82%D1%80%D0%B0%D0%B8%D0%B2%D0%B0%D0%B5%D0%BC-%D0%B4%D0%BE%D1%81%D1%82%D1%83%D0%BF-%D0%BA-%D1%81%D0%B5%D1%82%D0%B8

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In fact, doctoc works more correctly, but your extension is much more convenient, and I love it a lot. I’d like you have corrected those issues. Thanks in advance!

[BeeeWall]

I’ll take a look at doing that, then.

[BeeeWall]

I’d be willing to take a crack at it.

I suppose githubCompatibility could be changed to something like slugifyMode, with the options github, gitlab, vscode? (I haven’t really worked with VS Code extensions before, so I don’t know whether limiting it to specific values is possible or not, but if not, defaulting to “vscode” could work)