`lua_{autocmd}` like attribute

It is useful to manage scripts tied to plugins as modules, even if they are not necessarily scripts that load in association with plugins. Attributes are provided to achieve this. The attributes `lua_start`, `lua_{startup}`, and `lua_{autocmd}` are meant to be loaded at startup, after startup, and when the specified auto command is fired, respectively. We do not consider this to be the best configuration.

No files, tests, or entry points are named. Start by tracing how the existing lua_start and lua_{startup} attributes are handled, then clarify the intended configuration for lua_{autocmd}; done should mean the specified autocmd loads the associated module as described.

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