up/down arrows should move cells up and down even if focussed inside the input

[chriddyp]

Expected

• additional test for behavior on arrow up and down
• additional test for behavior on first and last row of the table (similar to test: can edit last and update data on "enter" in dash_test.ts)

[Marc-Andre-Rivet]

@valentijnnieman updating expected tests based on Chris' research at #172 (comment) to cover both the new behavior and potential regressions on:

edit mode

5. ✅ arrowing left-right in edit mode changes the cursor by single character.
6. ✅ arrowing up-down in edit mode moves the cursor to the front or the back of the cells 

focused

• arrow up navigates to previous cell in the column (+ edge case in first row)
• arrow down navigates to next cell in the column (+ edge case in last row)
• arrow left navigates to previous cell in the row (+ edge case in first column)
• arrow right navigates to next cell in the row (+ edge case in last column)

edit

• arrow up moves cursor to end of input content
• arrow down moves cursor to start of input content
• arrow left/right moves caret one position in that direction
  (see input input.selectionEnd/selectionStart for tests)

Once we have this behavior correct it would be a natural continuation to fix

focussed mode

8. ✖️ typing text keeps the cell in focussed mode: after typing, arrowing moves the cell, it does not move the cursor