scroll keybindings: top, bottom, page up, page down, page fractional

[mitchellh]

Fixes #259
Related #227

This introduces a number of new keybindings, with default bindings on Mac and Linux:

• scroll_to_top scrolls to the top of the viewport
• scroll_to_bottom scrolls to the bottom of the viewport
• scroll_page_up and scroll_page_down scroll by one page (number of visible rows) up or down, respectively
• scroll_page_fractional takes a floating point argument and scrolls by visible_rows * fraction, i.e. 1 is equal to scroll_page_down but 0.5 would let you scroll a half page down.

Default keybindings on Mac:

• cmd+home=scroll_to_top
• cmd+end=scroll_to_bottom
• cmd+page_up=scroll_page_up
• cmd+page_down=scroll_page_down

Default keybindings on Linux:

• shift+home=scroll_to_top
• shift+end=scroll_to_bottom
• shift+page_up=scroll_page_up
• shift+page_down=scroll_page_down

Example customizing scroll_page_fractional (default unbound):

# Scroll a half page up
keybind = shift+page_up=scroll_page_fractional:-0.5

Demo:

CleanShot.2023-08-09.at.07.45.39.mp4

cc @andrewrk