Add alias methods to ProblemBuilder

Existing methods are confusing as they have singular name but take
plural argument.

Author: malczuuu

src/main/java/io/github/problem4j/core/AbstractProblemBuilder.java

@@ -193,7 +193,7 @@ public ProblemBuilder extension(String name, Object value) {
    * @return this builder instance for chaining
    */
   @Override
-  public ProblemBuilder extension(Map<String, Object> extensions) {
+  public ProblemBuilder extensions(Map<String, Object> extensions) {
     if (extensions != null && !extensions.isEmpty()) {
       extensions.forEach(
           (key, value) -> {
@@ -212,7 +212,7 @@ public ProblemBuilder extension(Map<String, Object> extensions) {
    * @return this builder instance for chaining
    */
   @Override
-  public ProblemBuilder extension(Problem.Extension... extensions) {
+  public ProblemBuilder extensions(Problem.Extension... extensions) {
     if (extensions != null && extensions.length > 0) {
       Stream.of(extensions)
           .filter(AbstractProblemBuilder::isExtensionValid)
@@ -228,7 +228,7 @@ public ProblemBuilder extension(Problem.Extension... extensions) {
    * @return this builder instance for chaining
    */
   @Override
-  public ProblemBuilder extension(Collection<? extends Problem.Extension> extensions) {
+  public ProblemBuilder extensions(Collection<? extends Problem.Extension> extensions) {
     if (extensions != null && !extensions.isEmpty()) {
       extensions.stream()
           .filter(AbstractProblemBuilder::isExtensionValid)

src/main/java/io/github/problem4j/core/ProblemBuilder.java

@@ -113,23 +113,33 @@ public interface ProblemBuilder {
    * @param extensions map of extension keys and values
    * @return this builder instance for chaining
    */
-  ProblemBuilder extension(Map<String, Object> extensions);
+  ProblemBuilder extensions(Map<String, Object> extensions);
+
+  /**
+   * Adds multiple custom extensions from varargs of {@link Problem.Extension}.
+   *
+   * @param extension array of extensions
+   * @return this builder instance for chaining
+   */
+  default ProblemBuilder extension(Problem.Extension extension) {
+    return extensions(extension);
+  }
 
   /**
    * Adds multiple custom extensions from varargs of {@link Problem.Extension}.
    *
    * @param extensions array of extensions
    * @return this builder instance for chaining
    */
-  ProblemBuilder extension(Problem.Extension... extensions);
+  ProblemBuilder extensions(Problem.Extension... extensions);
 
   /**
    * Adds multiple custom extensions from a collection of {@link Problem.Extension}.
    *
    * @param extensions collection of extensions
    * @return this builder instance for chaining
    */
-  ProblemBuilder extension(Collection<? extends Problem.Extension> extensions);
+  ProblemBuilder extensions(Collection<? extends Problem.Extension> extensions);
 
   /**
    * Builds an immutable {@link Problem} instance with the configured properties and extensions.
@@ -151,4 +161,49 @@ public interface ProblemBuilder {
    * @return a new {@link Problem} instance
    */
   Problem build();
+
+  /**
+   * Adds multiple custom extensions from a map.
+   *
+   * <p><b>Deprecated</b> due to confusing name as singular "extension" suggests adding a single
+   * extension, while the method actually adds multiple extensions from the provided map.
+   *
+   * @param extensions map of extension keys and values
+   * @return this builder instance for chaining
+   * @deprecated use {@link #extensions(Map)} instead
+   */
+  @Deprecated
+  default ProblemBuilder extension(Map<String, Object> extensions) {
+    return extensions(extensions);
+  }
+
+  /**
+   * Adds multiple custom extensions from varargs of {@link Problem.Extension}.
+   *
+   * <p><b>Deprecated</b> due to confusing name as singular "extension" suggests adding a single
+   * extension, while the method actually adds multiple extensions from the provided vararg.
+   *
+   * @param extensions array of extensions
+   * @return this builder instance for chaining
+   * @deprecated use {@link #extensions(Problem.Extension...)} instead
+   */
+  @Deprecated
+  default ProblemBuilder extension(Problem.Extension... extensions) {
+    return extensions(extensions);
+  }
+
+  /**
+   * Adds multiple custom extensions from a collection of {@link Problem.Extension}.
+   *
+   * <p><b>Deprecated</b> due to confusing name as singular "extension" suggests adding a single
+   * extension, while the method actually adds multiple extensions from the provided collection.
+   *
+   * @param extensions collection of extensions
+   * @return this builder instance for chaining
+   * @deprecated use {@link #extensions(Collection)} instead
+   */
+  @Deprecated
+  default ProblemBuilder extension(Collection<? extends Problem.Extension> extensions) {
+    return extensions(extensions);
+  }
 }