Allow specifying workspace-specific EnvironmentVariableCollection mutators

[Tyriar]

For example:

export interface EnvironmentVariableMutator {
	readonly type: EnvironmentVariableMutatorType;
	readonly value: string;
	readonly workspace: Workspace | undefined;
}

export interface EnvironmentVariableCollection extends Iterable<[variable: string, mutator: EnvironmentVariableMutator]> {
	replace(variable: string, value: string, workspace?: Workspace): void;
	append(variable: string, value: string, workspace?: Workspace): void;
	prepend(variable: string, value: string, workspace?: Workspace): void;
}

Context: microsoft/vscode-python#20492 (comment)