Add vscode devcontainer config

[EverybodyKurts]

Rationale

It should be very simple to set up a working development environment for the project. This pull request will lower the barrier to contribution from new developers as well as make it easier to replicate any errors or oddities that might occur.

VSCode's devcontainer feature is the closest to a turn-key solution I've found for creating portable and easy-to-setup developer environments. Github users can either choose to set up a Github codespace to work remotely in the cloud or pull down the repo and boot up the devcontainer environment in vscode.

[EverybodyKurts]

Hmm, the pipeline for testing in windows failed. A quick glance of the stack trace states:

Hint: Could not find a version in your paket.dependencies file, consider adding 'version 6.2.1' at the top of your dependencies file (D:\a\FSharp.Data\FSharp.Data\paket.dependencies).

In this pr, I updated the required dotnet version from 6.0 to 7.0. When inside a devcontainer running on net6, I could not get ionide to load... it's only when I bumped up the dotnet version could I get ionide to load.

Unfortunately, I think rolling back to net6 would break the vscode devcontainer environment.