Add ability to configure thousands separator for `formatPageNumber` and `formatTOCPageNumber`

[kunalray1993]

Description

In the static functions TCPDF_STATIC::formatPageNumber and TCPDF_STATIC::formatTOCPageNumber, the thousands separator is currently hard-coded as a dot (.):

public static function formatPageNumber($num) {
    return number_format((float)$num, 0, '', '.');
}

public static function formatTOCPageNumber($num) {
    return number_format((float)$num, 0, '', '.');
}

However, number formatting conventions vary by country or user preference.
For example:

• Many locales use a comma (,) as the thousands separator
• Some locales use a space ( )
• Some users may not want any separator at all

Since TCPDF is used globally, having a fixed separator limits customization and forces developers to manually override or modify the library.

Proposed Enhancement

Introduce a configurable thousands separator that can be set at runtime, for example via:

• A new private static property in TCPDF_STATIC
• A new constant or class setting
• A setter method such as TCPDF_STATIC::setThousandsSeparator($separator)

This value should be used internally by:

• TCPDF_STATIC::formatPageNumber
• TCPDF_STATIC::formatTOCPageNumber
• Any other related formatting routines

Why This Matters

• Allows localization and internationalization of generated PDFs
• Avoids the need to patch or fork TCPDF just to change number formatting
• Enhances flexibility for developers generating documents with custom formatting requirements

Suggested Code Adjustment

Example idea (not final):

private static $thousands_separator = '.';

public static function setThousandsSeparator($separator) {
    self::$thousands_separator = $separator;
}

public static function formatPageNumber($num) {
    return number_format((float)$num, 0, '', self::$thousands_separator);
}