Allow all path setting to have multiple elements

[annacrombie]

Also, substitute ${workspaceDirectory} in all string settings values.

This allows you to do something like this in your settings.json:

	"mesonbuild.languageServerPath": "${workspaceFolder}/Tools/bin/muon",
	"mesonbuild.mesonPath": ["${workspaceFolder}/Tools/bin/muon", "meson"],

• You can use a muon/meson path relative to the workspace folder (at $WORK we check it in to source control).
• You can use specify mesonPath as muon meson which allows muon to drop in without requiring vscode-meson to support muon's CLI and without writing a wrapper script that can be called without any arguments. muon meson supports all of the required commands and some of the introspection API, so you can already use it to view the target tree.

[annacrombie]

This is based on #278 so keeping it a draft until that PR is merged.