replacer api, unit tests, and benchmarks (#354)

* added abstracted replacer api, and both char and regex based implementations

* added test dependencies for jmh and junit

* added unit tests and benchmarks for the replacer implementations

* updated replacers to accept specific closure types, added test to verify malformed placeholder handling

* updated jmh to 1.23, updated junit to 5.6.2

src/main/java/me/clip/placeholderapi/replacer/Replacer.java

@@ -0,0 +1,33 @@
+package me.clip.placeholderapi.replacer;
+
+import me.clip.placeholderapi.PlaceholderHook;
+import org.bukkit.OfflinePlayer;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.function.Function;
+
+public interface Replacer
+{
+
+	@NotNull
+	String apply(@NotNull final String text, @Nullable final OfflinePlayer player, @NotNull final Function<String, @Nullable PlaceholderHook> lookup);
+
+
+	enum Closure
+	{
+		BRACES('{', '}'),
+		BRACKETS('[', ']'),
+		PERCENT('%', '%');
+
+
+		public final char head, tail;
+
+		Closure(final char head, final char tail)
+		{
+			this.head = head;
+			this.tail = tail;
+		}
+	}
+
+}