Add an tab-end padding character

[simonomi]

Adds an option whitespace.characters.tabend similar to tabpad, but for the last character of a tab.

[editor.whitespace.characters]
tab = "→"
tabpad = "·"
tabend = "←" # Tabs will look like "→··←" (depending on tab width)

By default, tabend is whatever tabpad is, so all current behavior is unchanged. My config uses the box-drawing characters ╶, ─, and ╴, which makes ╶──╴ (how pretty!).