feat: add pageUp/pageDown/home/end to SlickCellSelection, fixes #794

[ghiscoding]

• fixes Allow pageup/pagedown/home/end etc during selection #794
• adding "pageUp/pageDown/home/end" to SlickCellSelection when using selection with "Shift+{key}"
• add hasDataView() and getViewportRowCount() methods to SlickGrid and also switch methods getViewportHeight() and getViewportWidth() from protected to public methods
• add new example-spreadsheet-dataview.html demo to manually test updated Cell Selection feature

TODOs

• tested manually, it should expect the following:
  • doing pageDown twice then pageUp once should equal to 1 pageDown
  • doing Shift+Home from anywhere should select all cells from current position to first row
  • doing Shift+End from should select all cells from current position to last row
  • using Pagination and doing Shift+{key} should select cells in current page only and not outside of it
• add new spreadsheet Example to test Cell Selection with Pagination as detailed above
• add Cypress E2E tests

basic SlickGrid example

with DataView & Pagination example