Add isAt method to ImmutableStringReader and StringReader

src/main/java/com/mojang/brigadier/ImmutableStringReader.java

@@ -23,4 +23,6 @@ public interface ImmutableStringReader {
     char peek();
 
     char peek(int offset);
+
+    boolean isAt(char c);
 }

src/main/java/com/mojang/brigadier/StringReader.java

@@ -76,6 +76,11 @@ public class StringReader implements ImmutableStringReader {
         return string.charAt(cursor + offset);
     }
 
+    @Override
+    public boolean isAt(char c) {
+        return canRead() && peek() == c;
+    }
+
     public char read() {
         return string.charAt(cursor++);
     }
@@ -249,7 +254,7 @@ public class StringReader implements ImmutableStringReader {
     }
 
     public void expect(final char c) throws CommandSyntaxException {
-        if (!canRead() || peek() != c) {
+        if (!isAt(c)) {
             throw CommandSyntaxException.BUILT_IN_EXCEPTIONS.readerExpectedSymbol().createWithContext(this, String.valueOf(c));
         }
         skip();

src/test/java/com/mojang/brigadier/StringReaderTest.java

@@ -585,4 +585,15 @@ public class StringReaderTest {
             assertThat(ex.getCursor(), is(0));
         }
     }
+
+    @Test
+    public void isAt() {
+        final StringReader reader = new StringReader("abc");
+        assertThat(reader.isAt('a'), is(true));
+        assertThat(reader.isAt('x'), is(false));
+        reader.setCursor(2);
+        assertThat(reader.isAt('c'), is(true));
+        reader.skip();
+        assertThat(reader.isAt('c'), is(false));
+    }
 }