Added timezone support to the FormatVariable interface and Date formatter

[lmuhlha]

The Problem

grafana/grafana#106452

When using ${__to:date:M} or similar date formatting on __from/__to, the value was evaluated in the client's local timezone instead of respecting the dashboard's configured timezone (e.g., UTC). This breaks Prometheus queries like month() == bool vector(${__to:date:M}) across different timezones.

The Fix
Made changes to 3 files:

1. formatRegistry.ts - Added timezone support to the interface and formatter

• Added getTimeZone?(): TimeZone method to the FormatVariable interface
• Imported dateTimeForTimeZone from @grafana/data
• Updated the Date formatter to use dateTimeForTimeZone(timeZone, nrValue) when a timezone is available
• Falls back to the old behavior (dateTime(nrValue)) when no timezone is provided

2. timeMacros.ts - Implemented timezone retrieval for time macros

• Added getTimeZone() method to TimeFromAndToMacro class
• This method retrieves the timezone from the scene's time range using getTimeRange(this._sceneObject).getTimeZone()

3. timeMacros.test.ts - Added tests

• Test 1: Verifies UTC timezone is respected
• Test 2: Verifies custom timezones (America/New_York) are respected
• Test 3: Verifies the specific bug case - cross-timezone month boundaries (March 31st 23:00 UTC appears as April 1st in Stockholm UTC+2)

[CLAassistant]

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