add method to get preferred web instance out of link

this is a preference that can be put in the link

src/open/clients/Element.js

@@ -88,4 +88,9 @@ export class Element {
 	canInterceptMatrixToLinks(platform) {
 		return platform === Platform.Android;
 	}
+
+    getPreferredWebInstance(link) {
+        const idx = trustedWebInstances.indexOf(link.webInstances[this.id])
+        return idx === -1 ? undefined : trustedWebInstances[idx];
+    }
 }

src/open/clients/Fractal.js

@@ -48,4 +48,6 @@ export class Fractal {
             return [new FlathubLink("org.gnome.Fractal")];
         }
     }
+
+    getPreferredWebInstance(link) {}
 }

src/open/clients/Nheko.js

@@ -50,4 +50,6 @@ export class Nheko {
             return [new FlathubLink("io.github.NhekoReborn.Nheko")];
         }
     }
+
+    getPreferredWebInstance(link) {}
 }

src/open/clients/Quaternion.js

@@ -47,4 +47,6 @@ export class Quaternion {
             return [new FlathubLink("com.github.quaternion")];
         }
     }
+
+    getPreferredWebInstance(link) {}
 }

src/open/clients/Tensor.js

@@ -47,4 +47,6 @@ export class Tensor {
             return [new FDroidLink("io.davidar.tensor")];
         }
     }
+
+    getPreferredWebInstance(link) {}
 }

src/open/clients/Weechat.js

@@ -46,4 +46,6 @@ export class Weechat {
     }
 
 	getInstallLinks(platform) {}
+
+    getPreferredWebInstance(link) {}
 }