Add default development container

[icanhazcheeze]

When you try GitHub codespaces beta, you start with a default build that doesn't include rustup or cargo. Having a .devcontainer file enables you to have a codespace with rust included in the image.

[icanhazcheeze]

Oops, quite new to the helix. I also added a commit than enables entering insert mode when you have a series of commands. I didn't intend to push it along with the other commit but forgot to branch before committing.

[the-mikedavis]

Could you drop out 930b4a5 since it's unrelated?

[the-mikedavis]

I'm not sure I'm against having a codespaces setup but it does seem odd to have for helix since codespaces tend to promote VS Code. That's not necessarily a bad thing but there is a particular savvy in being able to develop helix using helix

[icanhazcheeze]

Ok I dropped the other commit from master. Would you be happy to accept it in a different pull request?

[the-mikedavis]

Sorry for the delay. For now we're not interested in having a codespace configuration.

The fix from the dropped commit is being handled in #2634