Add support for toolchain installed using Swiftly

[nicolas-christe]

Describe the solution you'd like

Now that swiftly is available to install toolchains, it would be nice if the vscode extension can be configured to use the toolchain specified by .swift-version file in the root of the project

[plemarquand]

This should be addressed by #1478 when it merges.

[plemarquand]

#1478 is now merged