Add check and alert for larger JSON files

[acupoftee]

This PR addresses Issue #22 from the original repo.

• My approach for this iteration was to add an alert for large JSON files. This would be good feedback for the user when trying to parse JSON.

// First, check if it's plain text and exit if not
  const plainText = getTextFromTextOnlyDocument();
  if (!plainText || plainText.length > 3000000) {
    // If there is plain text and it's over 3MB, send alert
    plainText && alert('JSON Formatter Error: Cannot parse JSON larger than 3MB');
    port.disconnect();
    return;
  }

• Since issue #67 from the same included a possible solution for parsing large JSON files, this may be included in the next iteration