"nonpcinteraction" and "walkpastnpcs"

[MrDoubleA232]

Renames the "nonpccollision" field that Sarn added to "nonpcinteraction" for consistency with the player field, and gives it a corresponding config setting. Also makes it so that NPCs with it set will ignore block-like NPCs too.

Adds "walkpastnpcs", which specifically controls the bumping behaviour between two NPCs. It can have three values:

• 0: turns around as normal
• 1: other NPCs will bump off of it as usual, but it won't turn around
• 2: ignored entirely

Side note: this technically comes with a minuscule change to how fireballs and ice balls (and bullet bills modified via config) bump off of things. But it doesn't seem to make a practical difference.

[Supermario1313]

Just had an idea, even though config files don't support enums yet, it would still be nice to create one for walkpastnpcs to make the C++ code more readable (named constants are better than magic numbers) and export its values to Lua for users who want to read or write to this config field from a script.