Multi-root workspace support

Let's move toward support multi-root workspaces. What is this going to entail?

- Remove `mesonbuild.buildFolder` in favor of `mesonbuild.buildDirectories` where the value can either be a string or an object where the keys are the names of workspace folders and the values are their build directories.
- How will this work with the muon formatter/linter?
- How will this work with the language server?

cc @lukester1975 @wolfpld @xclaesse

Start by tracing how the mesonbuild.buildFolder setting is handled and where workspace folders are discovered. Then assess the requested buildDirectories mapping alongside the muon formatter/linter and language server. The work is done when multi-root behavior and its impact on each of those integrations are defined and supported.

Written by the indexing model from the issue text.