Focus in and out of the search field of `gum filter`

[simjnd]

Is your feature request related to a problem? Please describe. Use gum filter, unlike other components you can't scroll using standard keybindings (HJKL). Shortcuts like g or G don't work. SPACE can't be used to toggle selection when it is the only way to do it in gum choose.

Describe the solution you'd like Mentioned in #198, enable focusing in and out of the search field of gum filter:

• Pressing S focuses in the search field
  • Letter keys and spaces are typed in the search field
  • TAB toggles selection
  • Arrow keys can be used for scrolling
• Pressing ESC focuses out of the search field
  • HJKL and Arrow keys can be used for scrolling
  • g for jumping to top, G for going to bottom
  • TAB and SPACE both toggle selection