TypeScript: Ensure KernelEnum includes linear (#4441)

Author: BayanBennett

lib/index.d.ts

@@ -1784,6 +1784,7 @@ declare namespace sharp {
     interface KernelEnum {
         nearest: 'nearest';
         cubic: 'cubic';
+        linear: 'linear';
         mitchell: 'mitchell';
         lanczos2: 'lanczos2';
         lanczos3: 'lanczos3';