[json_syntax_generator] Separate the runtime library

[dcharkes]

Currently, the runtime library for the generated syntax classes is copied into each generated syntax.

We could consider making a separate runtime library package:

• All generated code would import this package.
• This packages would need to be published on pub.dev in order for hooks_runner to be able to roll into flutter_tools.

This would have the following benefits:

• No copy pasted in helper functions in every generated file: smaller diffs, and no diffs in unrelated syntax files.
• Properly sharing base classes such as JsonObjectSyntax.

However, publishing a separate package would lead to more churn:

• It would make the generated code less self-contained, leading to having to do an extra publish step on rolls.
• More process to avoid version skew between the generated code and runtime library when generating.

Currently, json_syntax_generator has many rough edges and many breaking changes, so we keep the helper library to be copy pasted into every generated file for now.