Adding MOVEMENT_EXERTION_MODIFIER as enchantment (#72580)

* First commit, praise Clang

* Update src/avatar_action.cpp

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Added documentation

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

doc/MAGIC.md

@@ -859,6 +859,7 @@ Character status value  | Description
 `MOVECOST_FLATGROUND_MOD`| How many moves you spend to move 1 tile on flat ground; shown in UI
 `MOVECOST_OBSTACLE_MOD` | How many moves you spend to move 1 tile, if this tile has a movecost more than 105 moves; not shown in UI
 `MOVECOST_SWIM_MOD`     | How many moves you spend to move 1 tile in water; not shown in UI
+`MOVEMENT_EXERTION_MODIFIER` | Affects how much physical exertion (activity_level) is required for the player to move a single tile. The value this affects is technically a float, so addition and multiplication operations can be performed on it, but player-facing effects only happen at specific whole-integer breakpoints. This can most easily be seen in `\data\json\ui\activity.json`.
 `NIGHT_VIS`             | How well you can see in darkness.  `ADD` adds tiles, so `"ADD": 3` increases night vision distance by 3 tiles.
 `OBTAIN_COST_MULTIPLIER`| Modifier for pulling an item from a container, as a handling penalty or bonus. `"add": 100` add 100 additional moves to item wield (1 second)
 `OVERKILL_DAMAGE`       | multiplies or contributes to the damage to an enemy corpse after death. The lower the number, the more damage caused.

src/avatar_action.cpp

@@ -222,7 +222,8 @@ bool avatar_action::move( avatar &you, map &m, const tripoint &d )
 
     // by this point we're either walking, running, crouching, or attacking, so update the activity level to match
     if( !is_riding ) {
-        you.set_activity_level( you.current_movement_mode()->exertion_level() );
+        you.set_activity_level( you.enchantment_cache->modify_value(
+                                    enchant_vals::mod::MOVEMENT_EXERTION_MODIFIER, you.current_movement_mode()->exertion_level() ) );
     }
 
     // If the player is *attempting to* move on the X axis, update facing direction of their sprite to match.

src/magic_enchantment.cpp

@@ -192,6 +192,7 @@ namespace io
             case enchant_vals::mod::CONSUME_TIME_MOD: return "CONSUME_TIME_MOD";
             case enchant_vals::mod::SWEAT_MULTIPLIER: return "SWEAT_MULTIPLIER";
             case enchant_vals::mod::STAMINA_REGEN_MOD: return "STAMINA_REGEN_MOD";
+            case enchant_vals::mod::MOVEMENT_EXERTION_MODIFIER: return "MOVEMENT_EXERTION_MODIFIER";
             case enchant_vals::mod::NUM_MOD: break;
         }
         cata_fatal( "Invalid enchant_vals::mod" );

src/magic_enchantment.h

@@ -170,6 +170,7 @@ enum class mod : int {
     CONSUME_TIME_MOD,
     SWEAT_MULTIPLIER,
     STAMINA_REGEN_MOD,
+    MOVEMENT_EXERTION_MODIFIER,
     NUM_MOD
 };
 } // namespace enchant_vals