[3/y] Fix unavailable generic protocol mock initializer

[andrewchang-bird]

Stack:
📚 #254 [6/y] Update example projects
📚 #253 [5/y] Improve support for configuring SPM Xcode projects
📚 #252 [4/y] Show help message no mockable types are generated
📚 #251 ← [3/y] Fix unavailable generic protocol mock initializer
📚 #250 [2/y] Fix generator caching for multi-project setups
📚 #249 [1/y] Optimize dependency graph traversal
📚 #245 Replace SwiftPM with Swift Argument Parser

Mock initializer functions were not migrated to use the new templates in 0.18. This caused unavailable generic protocols (e.g. inheriting from an opaque external type) to generate malformed output.

Before

// error: Consecutive statements on a line must be separated by ';'
public enum InheritingMissingExternalClassGeneric<T> {
}@available(*, unavailable, message: "'InheritingMissingExternalClassGeneric' inherits from the externally-defined type 'CBCentralManager' which needs to be declared in a supporting source file")

After

public enum InheritingMissingExternalClassGeneric<T> {
}
@available(*, unavailable, message: "'InheritingMissingExternalClassGeneric' inherits from the externally-defined type 'CBCentralManager' which needs to be declared in a supporting source file")