Refactor vswhere.exe integration (#104133)

`VisualStudio` calls `vswhere.exe` to find Visual Studio installations and determine if they satisfy Flutter's requirements. Previously, `VisualStudio` stored the JSON output from `vswhere.exe` as `Map`s, resulting in duplicated logic to read the JSON output (once to validate values, second to expose values). Also, `VisualStudio` stored two copies of the JSON output (the latest valid installation as well as the latest VS installation).

This change simplifies `VisualStudio` by introducing a new `VswhereDetails`. This type contains the logic to read `vswhere.exe`'s JSON output, and, understand whether an installation is usable by Flutter. In the future, this `VswhereDetails` type will be used to make Flutter doctor resilient to bad UTF-8 output from `vswhere.exe`.

Part of https://github.com/flutter/flutter/issues/102451.