Instead of deciding provider status in eval_has_provider, move the
decision to the provider Vim scripts.
Previously, provider loading worked as follows:
1. eval_has_provider() verified provider availability by searching for
the provider#providername#Call function and cached this verificaion as a static
variable for some providers
2. providers short-circuited on loading to prevent the definition of the
Call function (with the exception of the node provider that did not)
This commit changes the expected interface between nvim and its
providers to facilitate provider reloading, by splitting the
verification of the provider from the availability of the Call function.
eval_has_provider() now checks for a provider#providername#enabled
variable. It is up to the provider script to set this to 0 or 1
accordingly. eval_call_provider() remains unchanged.
All providers hosting a Call function were updated to respect this.
The clipboard provider now has a Reload function to reload the
provider.
