clock: add {month_name} placeholder with standalone month support

[yars068]

This adds a support for POSIX %OB modifier of strftime() for clock/calendar module through a new placeholder {month_name}.

Introduce a new {month_name} placeholder for the clock module tooltip. Unlike the standard library formatting which wraps month names in a date context (often forcing genitive case in inflected languages like Slavic, Baltic, or Finno-Ugric), this placeholder utilizes the POSIX %OB modifier to retrieve the standalone (nominative) month name.

It can resolve #5241, #3890.

Mine clock module config section looks as follows:

    "clock": {
        "format"  : "{:%H:%M%n%d.%m.%Y}",
        "justify" : "center",
        "tooltip-format": "<span color='#ecc6d9'><b>{month_name} {:%Y}</b></span>\n<tt><small>{calendar}</small></tt>",
        "calendar": {
            "mode"          : "month",
            "mode-mon-col"  : 3,
            "weeks-pos"     : "right",
            "on-scroll"     : 1,
            "format": {
                "months":     "", //"<span color='#ffead3'><b>{}</b></span>",
                "days":       "<span color='#ecc6d9'><b>{}</b></span>",
                "weeks":      "<span color='#ffcc66'><b>Н.{}</b></span>",
                "weekdays":   "<span color='#ffcc66'><b>{}</b></span>",
                "today":      "<span color='#00a000'><b><u>{}</u></b></span>"
            },
        },
    },