Add experimental support for JPEG-XL, requires libvips with libjxl

The prebuilt binaries do not include support for this format.

test/unit/jxl.js

@@ -0,0 +1,97 @@
+'use strict';
+
+const assert = require('assert');
+
+const sharp = require('../../');
+
+describe('JXL', () => {
+  it('called without options does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().jxl();
+    });
+  });
+  it('valid distance does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().jxl({ distance: 2.3 });
+    });
+  });
+  it('invalid distance should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ distance: 15.1 });
+    });
+  });
+  it('non-numeric distance should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ distance: 'fail' });
+    });
+  });
+  it('valid quality > 30 does not throw an error', () => {
+    const s = sharp();
+    assert.doesNotThrow(() => {
+      s.jxl({ quality: 80 });
+    });
+    assert.strictEqual(s.options.jxlDistance, 1.9);
+  });
+  it('valid quality < 30 does not throw an error', () => {
+    const s = sharp();
+    assert.doesNotThrow(() => {
+      s.jxl({ quality: 20 });
+    });
+    assert.strictEqual(s.options.jxlDistance, 9.066666666666666);
+  });
+  it('valid quality does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().jxl({ quality: 80 });
+    });
+  });
+  it('invalid quality should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ quality: 101 });
+    });
+  });
+  it('non-numeric quality should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ quality: 'fail' });
+    });
+  });
+  it('valid decodingTier does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().jxl({ decodingTier: 2 });
+    });
+  });
+  it('invalid decodingTier should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ decodingTier: 5 });
+    });
+  });
+  it('non-numeric decodingTier should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ decodingTier: 'fail' });
+    });
+  });
+  it('valid lossless does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().jxl({ lossless: true });
+    });
+  });
+  it('non-boolean lossless should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ lossless: 'fail' });
+    });
+  });
+  it('valid effort does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().jxl({ effort: 6 });
+    });
+  });
+  it('out of range effort should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ effort: 10 });
+    });
+  });
+  it('invalid effort should throw an error', () => {
+    assert.throws(() => {
+      sharp().jxl({ effort: 'fail' });
+    });
+  });
+});