provider: Major refactor

- Providers for features are now registered as a unit. For example, instead of
  calling `register_provider("clipboard_get")` and
  `register_provider("clipboard_set")`, clients call
  `register_provider("clipboard")` and nvim will assume it implements all
  methods of the "clipboard" feature
- Bootstrapping code was removed. With the `api_spawn` function exposed to
  vimscript, it's no longer necessary and will be handled by plugins
  distributed with nvim.
- Now the `has` function will return true if there's a live channel that
  has registered as a provider for the feature.
- 'initpython'/'initclipboard' options were removed
- A new API function was exposed: `vim_discover_features` which returns an
  object with information about pluggable features such as 'python' or
  'clipboard'

src/nvim/api/vim.c

@@ -502,22 +502,28 @@ 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.
+/// Registers the channel as the provider for `feature`. This fails if
+/// a provider for `feature` is already provided by another channel.
 ///
 /// @param channel_id The channel id
-/// @param method The method name
+/// @param feature The feature name
 /// @param[out] err Details of an error that may have occurred
-void vim_register_provider(uint64_t channel_id, String method, Error *err)
+void vim_register_provider(uint64_t channel_id, String feature, Error *err)
 {
   char buf[METHOD_MAXLEN];
-  xstrlcpy(buf, method.data, sizeof(buf));
+  xstrlcpy(buf, feature.data, sizeof(buf));
 
   if (!provider_register(buf, channel_id)) {
-    set_api_error("Provider already registered", err);
+    set_api_error("Feature doesn't exist", err);
   }
 }
 
+/// Returns a feature->method list dictionary for all pluggable features
+Dictionary vim_discover_features(void)
+{
+  return provider_get_all();
+}
+
 /// 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.