Add simplified namespace access to generated script types

[ImmutableOctet]

We should be able to write something like:

start_thread<player_jump_intro>(); // Starts `player_jump_intro` script in current state.

from e.g. a player_common script in the same folder.

Some prerequisites I can think of are:

• Script headers need to be separate generated files, rather than embedded into the script's TU. (Should be supported already, just needs to be configured)
• Usable/human-readable type aliases should be provided from script headers (We have mangled names currently, but underlying script_t should be made accessible in some way)
• Consumption headers need to be generated and included properly for each script folder (Should be configuration related; same as declaration headers)
• We may need to tweak a few things to ensure the script-fiber's bootstrap process works as expected.