fix(types): Fix the signature of the tick format callback for timeseries

[stof]

Issue

Details

When running typescript in strict modes, it checks the compatibility of callback signatures.
When using a timeseries axis and passing a date formatting callback for the ticks, it currently returns that error:

assets/js/widget/program-evolution-chart.js:76:15 - error TS2322: Type '(arg0: Date) => string' is not assignable to type 'string | ((this: Chart, x: number | Date) => string | number) | ((this: Chart, index: number, categoryName: string) => string) | undefined'.
  Type '(arg0: Date) => string' is not assignable to type '(this: Chart, x: number | Date) => string | number'.
    Types of parameters 'arg0' and 'x' are incompatible.
      Type 'number | Date' is not assignable to type 'Date'.
        Type 'number' is not assignable to type 'Date'.

76       format: formatDate,
                 ~~~~~~~~~~

The documentation explicitly says that timeseries axis are always passing a Date there, so there is no need to require a callback support both dates and numbers:

[coveralls]

Coverage increased (+0.06%) to 90.526% when pulling cfdf461 on stof:fix_tick_format into 9a87464 on naver:master.

[netil]

@stof thanks for the contribution!