Added methods for singular and plural locale mappings

Introduced `displayNamesSingular()` and `displayNamesPlural()` methods in `Currency.Builder`. These provide unmodifiable mappings of singular and plural display names for various locales, improving extensibility and localization support.

Author: NonSwag

src/main/java/net/thenextlvl/service/api/economy/currency/Currency.java

@@ -5,9 +5,11 @@
 import net.kyori.adventure.text.Component;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.Contract;
+import org.jetbrains.annotations.Unmodifiable;
 import org.jspecify.annotations.NullMarked;
 
 import java.util.Locale;
+import java.util.Map;
 import java.util.Optional;
 import java.util.OptionalInt;
 
@@ -46,7 +48,6 @@ default Component getDisplayNameSingular(Audience audience) {
      */
     Component getDisplayNameSingular(Locale locale);
 
-
     /**
      * Retrieves the plural display name component of the currency based on the audience's locale.
      * <p>
@@ -109,6 +110,15 @@ default Component format(Number amount, Audience audience) {
      * singular and plural display names, currency symbol, and fractional digits.
      */
     interface Builder {
+        /**
+         * Retrieves a map containing the singular display names of the currency for various locales.
+         *
+         * @return an unmodifiable map of {@code Locale} and {@code Component} objects
+         * representing the singular display names of the currency.
+         */
+        @Unmodifiable
+        Map<Locale, Component> displayNamesSingular();
+
         /**
          * Sets the singular display name of the currency for a specific locale.
          *
@@ -128,6 +138,15 @@ interface Builder {
         @Contract(value = "_ -> new")
         Optional<Component> displayNameSingular(Locale locale);
 
+        /**
+         * Retrieves a map containing the plural display names of the currency for various locales.
+         *
+         * @return an unmodifiable map of {@code Locale} and {@code Component} objects
+         * representing the plural display names of the currency.
+         */
+        @Unmodifiable
+        Map<Locale, Component> displayNamesPlural();
+
         /**
          * Sets the plural display name of the currency for a specific locale.
          *