Add possibility to use default tick mark formatter implementation as a fallback

[kirchet]

Is your feature request related to a problem? Please describe.

For now we can provide tickMarkFormatter to format time axis tick marks.

lightweight-charts/src/model/time-scale.ts

Lines 192 to 197 in 1afc8db

	/**
	* Tick marks formatter can be used to customize tick marks labels on the time axis.
	*
	* @defaultValue `undefined`
	*/
	tickMarkFormatter?: TickMarkFormatter;

lightweight-charts/src/model/time-scale.ts

Lines 78 to 93 in 1afc8db

	/**
	* The `TickMarkFormatter` is used to customize tick mark labels on the time scale.
	*
	* This function should return `time` as a string formatted according to `tickMarkType` type (year, month, etc) and `locale`.
	*
	* Note that the returned string should be the shortest possible value and should have no more than 8 characters.
	* Otherwise, the tick marks will overlap each other.
	*
	* @example
	* ```js
	* const customFormatter = (time, tickMarkType, locale) => {
	* // your code here
	* };
	* ```
	*/
	export type TickMarkFormatter = (time: Time, tickMarkType: TickMarkType, locale: string) => string;

But we cannot override the behaviour for some special cases and call the default implementation otherwise

Describe the solution you'd like

We can change the signature in the following way:

export type TickMarkFormatter = (time: Time, tickMarkType: TickMarkType, locale: string) => string | null;

In case when this funtion returns null we can use the default tick mark formatter implementation.