Add Isabelle Extension

[lovercast]

Add support for the Isabelle proof assistant extension makarius/Isabelle2021-1 which is available from the Microsoft Visual Studio Marketplace.

[filiptronicek]

Hey there @lovercast 👋, thanks for reaching out about the Isabelle extension. Could you please first contact the author about them publishing the extension themselves?

If you did that already please let me know. I was not able to find the source code but maybe we just need to dig a bit deeper.

[lovercast]

Hi! Thanks for responding.
The code for the extension is available on the Mercurial and also on the Git mirror.