Feature: allow scrolling 1 day at the time

[pdpino]

PR overview - Change logic to allow scrolling one day at the time. This fixes multiple issues:

• Fixes Select number of days scrolled #128:
  • use prop allowScrollByOneDay={true} to scroll by days instead of by pages
• Fixes wrong date with fixedHorizontally #141 (see this comment)
  • Adds the prop pageStartAt, that allows customizing the date shown in the left
  • Breaking change: prop weekStartsOn is removed. Instead, use prop pageStartAt.weekday (README has new prop details)
• Fixes Selected Date Position. #185:
  • add left option to goToDate(date, { left: number }) --> the date will show at number of days from the left. i.e. Use left = 0 to show the target date at the most left
  • Breaking change: second parameter is no longer animated, now is an object { animated = true, left = null } (README has signature details)
• Fixes goToNextDay & goToPreviousDay #186:
  • add goToNextDay() and goToPrevDay() methods
• Fixes BUG: UI Drag #258:
  • I confirm this bug does not occur, users need to set allowScrollByOneDay={true} and it should work
  • (Though the issue may still occur in v0.16.0)

[pdpino]

Small code walkthrough - (in case is useful for reviewing the PR 🙂)

• the relevant state for horizontal pages scrolling is:
  • this.state.initialDates
  • this.currentPageIndex
  • dayOffset --> this one is new, useful to scroll by less than 1-page at the time
  • this.state.currentMoment (always derivated from the above ones)
• Note: dayOffset is not persisted (is re-computed from currentMoment if necessary)
• Code changes to support day offset:
  • onMomentumScrollEnd callback only minor changes
  • goToDate(date) computes target page-index and target day-offset (handles user options, offset overflow)
  • goToPageIndex() supports navigating to a page-index and day-offset
    • also needs to support old behavior with dayOffset = null | undefined
    • index and offset are handled via FlatList.scrollToIndex({ index, viewOffset, ... })

[hoangnm]

It looks great

[rdewolff]

Amazing! Thanks for your hard work 🥳

[rdewolff]

Does the main README require some update?
We should show off all the new features in there too :)