Handle Swiftly toolchain not installed error #1852
Description
Is your feature request related to a problem? Please describe.
When Swiftly does not have an appropriate toolchain to use with the version provided by a .swift-version file, it will fail with an error message:
The swift version file `<redacted>/Documents/swiftlang/swiftly/.swift-version` uses toolchain version 6.1.2, but it doesn't match any of the installed toolchains. You can install the toolchain with `swiftly install`.
When this happens, the Swift extension will simply bail and display the error message. I have to manually fix this by running swiftly install in my terminal and then restart VS Code.
Describe the solution you'd like
The Swift extension should detect this failure condition and automatically run the Swiftly command to get me up and running. It should also ask before doing so.