Introducing a minimal devcontainer (for codespaces)

[mtzguido]

This PR adds the .devcontainer directory with a specification for how to create an F* devcontainer. This devcontainer is what's used to generate a 'codespace', which is just a devcontainer running on Github's servers.

The container is based on a Dockerfile that installs our usual dependencies, and will also run make as part of the creation process. Also, it automatically installs the F* vscode extension, so starting a codespace will give a usable F* out of the box.

The container can also be run locally within vscode by just using the "Reopen in container" command. VS Code will in fact automatically suggest that when opening the workspace.

I tried to base this container on top of Github's codespace image (instead of a plain Ubuntu image), but I got different behaviors when running locally and on codespaces.. so I dropped that idea for now.

Note: when someone creates a codespace from the F* repo, the resource consumption (/ billing) runs on their tab. Having this file here should not use any resources from the FStarLang org.

[mtzguido]

BTW you can try this PR out by hitting the '<>Code' button above.

It will take a while to build but you should just see a VS code UI when it is done.

[nikswamy]

Wow, this is really nice. I was able to get a working environment with VS Code just clicking on the code button.

One nit: once the editor launched, I had to reload the window to get syntax highlighting and gutter icons. Not sure why. We might record that in a README somewhere as FAQ/workaround.