[Feature] Add final tick number as third parameter to axis label formatter

### What problem does this feature solve?

**Case**

Rather than show a numerical value of the last (upper bound) label on the Y Axis, it is sometimes desirable to show something else programmatically.

**Context**

We had a goal to show a unit of measure (ex: kWh, m/s, or A) on the last Y axis label. Since assigning axis.splitNumber doesn't always linearly corelate to the number of ticks, we couldn't determine the last tick without risky code.

**Rationale**

By having the final tick number in the axis label formatter, we could determine what tick was the last one and easily write that logic within the formatter. Without this, we could only entertain a solution [as described in Stack Overflow here](https://stackoverflow.com/a/77102205), which not intended to be done, and requires that the chart actually be rendered before we apply the actual formatting that we want.

### What does the proposed API look like?

**Current** ([described in docs](https://echarts.apache.org/en/option.html#xAxis.axisLabel.formatter))

```
formatter: function (value, index) {
return value + 'kg';
}
```

**Proposed**

```
formatter: function (value, index, totalTicks) {
const desiredLastTickLabel = "desired value here"
const isLastTick = index === totalTicks - 1

return isLastTick ? desiredLastTickLabel : value + 'kg';
}