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

The prebuilt binaries do not include support for this format.

lib/constructor.js

@@ -308,6 +308,10 @@ const Sharp = function (input, options) {
     heifCompression: 'av1',
     heifEffort: 4,
     heifChromaSubsampling: '4:4:4',
+    jxlDistance: 1,
+    jxlDecodingTier: 0,
+    jxlEffort: 7,
+    jxlLossless: false,
     rawDepth: 'uchar',
     tileSize: 256,
     tileOverlap: 0,