Remove the isNum helper function

The call-sites are replaced by direct `typeof`-checks instead, which removes unnecessary function calls. Note that in the `src/`-folder we already had more `typeof`-cases than `isNum`-calls.

These changes were *mostly* done using regular expression search-and-replace, with two exceptions:
 - In `Font._charToGlyph` we no longer unconditionally update the `width`, since that seems completely unnecessary.
 - In `PDFDocument.documentInfo`, when parsing custom entries, we now do the `typeof`-check once.

src/shared/util.js

@@ -1034,10 +1034,6 @@ function isBool(v) {
   return typeof v === "boolean";
 }
 
-function isNum(v) {
-  return typeof v === "number";
-}
-
 function isString(v) {
   return typeof v === "string";
 }
@@ -1146,7 +1142,6 @@ export {
   isBool,
   IsEvalSupportedCached,
   IsLittleEndianCached,
-  isNum,
   isSameOrigin,
   isString,
   MissingPDFException,