provider: New module used to expose extension points for core services

Introducing the concept of providers: co-processes that talk with the editor
through the remote API and provide implementation for one or more core
services.

The `provider_register` function and it's API wrapper can be used by channels
that want to self-register as a service provider.

Some old builtin vim features will be re-implemented as providers. The
`provider_has_feature` function is used to check if a provider
implementing a certain feature is available(It will be called by the `has`
vimscript function to check for features in a vim-compatible way)

This implements the provider module without exposing any extension points, which
will be done in future commits.

src/nvim/api/vim.c

@@ -10,6 +10,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/api/buffer.h"
 #include "nvim/os/channel.h"
+#include "nvim/os/provider.h"
 #include "nvim/vim.h"
 #include "nvim/buffer.h"
 #include "nvim/window.h"
@@ -503,6 +504,22 @@ void vim_unsubscribe(uint64_t channel_id, String event)
   channel_unsubscribe(channel_id, e);
 }
 
+/// Registers the channel as the provider for `method`. This fails if
+/// a provider for `method` is already registered.
+///
+/// @param channel_id The channel id
+/// @param method The method name
+/// @param[out] err Details of an error that may have occurred
+void vim_register_provider(uint64_t channel_id, String method, Error *err)
+{
+  char buf[METHOD_MAXLEN];
+  xstrlcpy(buf, method.data, sizeof(buf));
+
+  if (!provider_register(buf, channel_id)) {
+    set_api_error("Provider already registered", err);
+  }
+}
+
 /// Writes a message to vim output or error buffer. The string is split
 /// and flushed after each newline. Incomplete lines are kept for writing
 /// later.