Add "Brightness" image effect

Looks pretty good, thank you!

A few minor points below:

+++ b/src/Plugin/ImageEffect/BrightnessImageEffect.php
@@ -0,0 +1,98 @@
+      '@level' => 0,
+    ));
+

I think it should be part of the if...elseif block just below. Not sure "±0%" is very meaningful, though.

+++ b/src/Plugin/ImageEffect/BrightnessImageEffect.php
@@ -0,0 +1,98 @@
+    }
+
+    elseif ($this->configuration['level'] < 0) {

Unnecessary blank line.

+++ b/src/Plugin/ImageToolkit/Operation/gd/Brightness.php
@@ -0,0 +1,50 @@
+  /**
+   * {@inheritdoc}
+   */
+  protected function arguments() {
+    return [
+      'level' => [
+        'description' => 'The brightness level.',
+      ],
+    ];
+  }
+

This is already in the trait, should be removed.

+++ b/src/Plugin/ImageEffect/BrightnessImageEffect.php
@@ -0,0 +1,96 @@
+    if ($this->configuration['level'] > 0) {
+      $level = $this->t('+@level%', array(
+        '@level' => $this->configuration['level'],
+      ));
+    }
+    elseif ($this->configuration['level'] < 0) {
+      $level = $this->t('-@level%', array(
+        '@level' => ltrim($this->configuration['level'], '-'),
+      ));
+    }
+    else {
+      $level = drupal_placeholder($this->t('Unchanged'));
+    }

drupal_placeholder is deprecated so I think we should avoid using it. I think you could use $this->t('%level', ['%level' => $this->t('Unchanged')]); but I did not try it. also you can simplify a bit

    if ($this->configuration['level'] != 0) {
      $level = $this->configuration['level'] > 0 ? $this->t('+@level%', ['@level' => $this->configuration['level']]) : $this->t('@level%', ['@level' $this->configuration['level']]);
    }
    else ...

Using t() with just a placeholder is IMHO also bad practice. If anything, then use SafeMarkup::format()/FormattableMarkup.

Alternatively -

a) introduce a twig template to do the formatting
b) just do $level = $this->t('Unchanged'); which is the simplest, you won't be italicizing it but I do not think it's a big deal

Looks great. One last thing before RTBC

+++ b/image_effects.module
@@ -25,6 +25,10 @@ function image_effects_theme() {
+    // Brightness image effect - summary
+    'image_effects_brightness_summary' => [
+      'variables' => ['data' => NULL],
+    ],

I think we should also have the effect in the variables, see the

     'image_effects_watermark_summary' => [
       'variables' => ['data' => NULL, 'effect' => []],
     ...

In fact it looks like the color_shift summary is wrong :(
The core's effects all have an 'effect' key.

Looks OK to me now, RTBC

Additionally I will post a little follow up issue for the wrong color shift summary implementation

Thanks a bunch - in fact I am the culprit here ;)

Looks good to me too.

Committed. Thank you!