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/ccitt_stream.js

@@ -48,7 +48,7 @@ var CCITTFaxStream = (function CCITTFaxStreamClosure() {
 
   CCITTFaxStream.prototype.readBlock = function() {
     while (!this.eof) {
-      let c = this.ccittFaxDecoder.readNextChar();
+      const c = this.ccittFaxDecoder.readNextChar();
       if (c === -1) {
         this.eof = true;
         return;