Restore Loom mappings and modImplementation for Fabric deps

Loom needs an explicit mappings entry, and Fabric loader / API must be
declared with modImplementation so they get remapped against the active
mappings; the previous commit dropped both, which left the build with
no mappings at all and unremapped Fabric jars. Reference the
loom_version property in the plugin block too so it tracks
gradle.properties.

build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'net.fabricmc.fabric-loom' version '1.17-SNAPSHOT'
+    id 'net.fabricmc.fabric-loom' version "${project.loom_version}"
     id 'maven-publish'
 }
 
@@ -30,8 +30,9 @@ repositories {
 
 dependencies {
     minecraft "com.mojang:minecraft:${project.minecraft_version}"
-    implementation "net.fabricmc:fabric-loader:${project.loader_version}"
-    implementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
+    mappings loom.officialMojangMappings()
+    modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
+    modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
 }
 
 processResources {