Enable the ESLint prefer-const rule in the web/ directory

Please find additional details about the ESLint rule at https://eslint.org/docs/rules/prefer-const

Note that this patch is generated automatically, by using the ESLint `--fix` argument, and will thus require some additional clean-up (which is done separately).

web/.eslintrc

@@ -9,5 +9,6 @@
 
     // ECMAScript 6
     "no-var": "error",
+    "prefer-const": "error",
   },
 }

web/annotation_layer_builder.js

@@ -64,7 +64,7 @@ class AnnotationLayerBuilder {
         return;
       }
 
-      let parameters = {
+      const parameters = {
         viewport: viewport.clone({ dontFlip: true }),
         div: this.div,
         annotations,