Allow tab virtual whitespace padding

[A-Walrus]

Resolves #3427.
Added new whitespace.characters.tabpad option which sets the padding for tabs
Example:

[the-mikedavis]

I think this is a good angle implementation-wise but it may be preferrable to instead have the config break down like

[editor.whitespace]
render = "all"
characters.tab = { character = "→", padding = " " } # default values
# also accept the old syntax for backwards compatibility and simplicity:
characters.tab = "→"

# or alternatively
[editor.whitespace.characters]
tab = "→"
tabpad = "·" # using kakoune's name but we should probably use something more clear

I think this would be preferrable (albeit a larger change) because the characters could be represented by chars instead of a String - so no allocations - and there isn't a chance of misconfiguring the padding in a confusing way. With the current approach, you could use not enough padding in your configuration and end up being confused why a file with 8-column-width tabs only shows interpuncts for a few of the columns.