GLFM slash commands in Source Editor

Problem to solve

The markdown editor implemented in issues, epics, and comments as a textarea has functionality built in to help users write GLFM. One of the most useful are the supported quick actions (slash commands).

In order to even consider the Source Editor as a suitable replacement for the textarea, we need to support these quick actions in a similar way. That includes the UI that shows up to aide the user in available commands and autocomplete as they type.

Proposal

Create an extension for Source Editor that can be loaded in the context of editing GLFM files and provide suggestions when the / command is invoked.

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