Forward extra environment variables to manifest evaluation

Users can now declare extra environment variables that Tuist forwards when evaluating Project.swift, Workspace.swift, Package.swift, and project description helpers. In Tuist.swift, add a manifestEnvironment list under GenerationOptions with exact names or prefix patterns ending in *. Matched variables are visible during manifest evaluation and any value change invalidates the manifest cache. This removes the need to prefix every variable with TUIST_ to make it available during tuist generate.