We should add a default workspace settings file for VSCode (one of the primary editors used by Flutter contributors) to ensure that it doesn't format on save by default while contributors are making changes to the Flutter repository.

It's frustratingly common that someone makes a good change, saves it, and then realizes to their horror that they have reformatted the entire class file. They then have to go through and revert the formatting changes while retaining the semantic changes they've made, a time-consuming and somewhat error-prone process. Adding this file to override the default VSCode settings will remove one tripwire for new contributors to the project.

While the .vscode folder is ignored using .gitignore today, adding an exclusion for settings.json is a best practice when it contains values that are intended to be enforced by a project (e.g. formatting). This is also documented by the VSCode team.