Allow extensions to add floating buttons within a text editor (similar to the "Add Configuration..." button in launch.json editors)

[djnicholson]

When you open a launch.json file in vscode you see a prominent "Add Configuration..." button in the lower right hand corner of the text editor.

It would be great if extensions could cause similar buttons to appear within text editors (only when editing files in a specific language).

The closest I have managed is to get is by creating a CodeLensProvider that provides a CodeLens at Range(0, 0, 0, 0). This only gets me a small link at the top of the document though and it is easy for the user to miss.

Related (unanswered) StackOverflow question: https://stackoverflow.com/questions/60171826/can-vscode-extensions-add-floating-buttons-within-a-text-editor-similar-to-ad
Related issue where this feature would be used in an extension: neo-project/neo-visual-tracker#49