feature request: add support for lmap and diagraph argument

[TheSast]

The {char} argument for the default implementation of f respects the user's lmap mappings and allows the user to use digraphs as the argument for the search. (It's better described at :h f)

For the latter feature, here is an example:

-- v
return loc == "de" and "Großstadt" or "Metropolis"

f<C-k>ss

--                         v
return loc == "de" and "Großstadt" or "Metropolis"

Without this it can become quite difficult to use f/t motions to navigate when writing languages other than english.