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/option_defs.h

@@ -631,8 +631,6 @@ EXTERN int p_write;             /* 'write' */
 EXTERN int p_wa;                /* 'writeany' */
 EXTERN int p_wb;                /* 'writebackup' */
 EXTERN long p_wd;               /* 'writedelay' */
-EXTERN char *p_ipy;             // 'initpython'
-EXTERN char *p_icpb;            // 'initclipboard'
 
 /*
  * "indir" values for buffer-local opions.