Add activation event for .adoc files

[ggrossetie]

Thanks @ohhmm ! I'm curious, does it solve an issue in a specific scenario?

[ohhmm]

Yes, here's STR:

• Specify this extension for .devcontainer/devcontainer.json in "extensions" array. This makes this extension installed into the devcontainer automatically upon opening the project.
• Peek an .adoc file. With this change, extension activated on opening.

[ggrossetie]

Awesome, thanks!