Enable the ESLint prefer-const rule globally (PR 11450 follow-up)

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

With the recent introduction of Prettier this sort of mass enabling of ESLint rules becomes a lot easier, since the code will be automatically reformatted as necessary to account for e.g. changed line lengths.

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).

src/core/image.js

@@ -195,7 +195,7 @@ var PDFImage = (function PDFImageClosure() {
             );
         }
       }
-      let resources = isInline ? res : null;
+      const resources = isInline ? res : null;
       this.colorSpace = ColorSpace.parse(
         colorSpace,
         xref,