isAlnum function is incorrect in mailmerge example: check only latin symbols

Expected Behavior

Provided sample mail_merge works as described. Really it works only for headers with latin symbols (e.g. English).

Sample URL: https://sites.google.com/site/scriptsexamples/custom-methods/create-text-from-template#TOC-Documentation
Description: The cause is isAlnum function. That is naively implemented as:

function isAlnum(char) {
  return char >= 'A' && char <= 'Z' ||
    char >= 'a' && char <= 'z' ||
    isDigit(char);
}

But Unicode standard describe much more "Alpha" symbols in different categories.

So really that function should be implemented like:

function isAlnum(char) {
  return !! char.match(/[\p{Number}\p{Letter}]/u)
}

and also function isDigit may be dropped (that also potentially has similar problems)

Actual Behavior

Headers on said in Russian or Korean language just ignored and values does not substituted in template.

Steps to Reproduce the Problem

1. Run example with column name, said in Russian or Korean language.

Open gmail/mailmerge/mailmerge.gs and inspect the isAlnum function referenced in the issue. Run the mailmerge example with Russian or Korean column headers, then verify that the updated character check substitutes those header values while preserving existing behavior for Latin headers.