feat: add variable substitution support for vscode's ${workspaceFolder:x}

[tomlau10]

resolves #2987

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implementation details: #2987 (comment)

• workspace folder names are captured from client's initialization request param workspaceFolders[].name
• if provided, it is then stored into the created workspace.scope object
• finally when resolving a ${workspaceFolder:x} path placeholder
  use the decoded uri of the workspace.scope object that having x as the folder name
  (which becomes absolute path of that folder)

Example

• a dummy _.code-workspace

{ "folders": [	{ "name": "root", "path": "."	},	{ "name": "client", "path": "client"	},	{ "name": "server", "path": "server"	}	], "settings": { "Lua.workspace.library": [ "${workspaceFolder:client}",	]	} }

• upon inspecting log file, the client is sending the following to server during init

 workspaceFolders = { { name = "root", uri = "file:///c%3A/Users/TomLau/test" }, { name = "client", uri = "file:///c%3A/Users/TomLau/test/client" }, { name = "server", uri = "file:///c%3A/Users/TomLau/test/server" } }

• therefore ${workspaceFolder:client} will be resolved as furi.decode('file:///c%3A/Users/TomLau/test/client')
  which becomes C:\Users\TomLau\test\client