feat: allow treating steel targets as practice targets

[chaosvolt]

Purpose of change

This allows you to treat steel targets as shootable like you can with practice target traps, via a new flag that can in turn be used for other targets should we opt to add them in the future.

Describe the solution

C++ changes:

1. In ranged.cpp, set target_ui::choose_initial_target so it also counts terrain or furniture with the SHOOT_ME flag as selectable targets too.

JSON changes:
2. Added SHOOT_ME flag to steel target furniture.

Documentation changes:

1. Documented new flag in json_flags.md

Describe alternatives you've considered

1. Hardcoding it so only specific furniture IDs can be shot at. That'd be kinda silly so meh.
2. Coding in support for traps having flags so that practice target traps can be JSONized too.

Testing

1. Checked affected JSON for syntax and lint errors.
2. Compiled and load-tested.
3. Spawned in a gun and a steel target, confirmed I can aim and shoot at it.
4. Confirmed doing so grants me EXP.
5. Spawned in a practice target too, aim function prioritizes practice target traps over steel targets.
6. Spawned in a debug monster, aim prioritizes the monster over either.

Additional context

Checklist

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