Add more precise TypeScript type for 'tickPlacement'

[dave-meyer]

According to the documentation, the tickPlacement property can have one of two values, 'between' and 'on'. However, the TypeScript type of ApexXAxis.tickPlacement is string. Should we limit the type to the only two possible values? Or are there any other possible values / use cases I am not aware of?

Currently:

type ApexXAxis = {
    ...
    tickPlacement?: string,
    ...
}

Possible change:

type ApexXAxis = {
    ...
    tickPlacement?: 'between' | 'on',
    ...
}

What do you guys think?