refactor(decorations): unglobal decor_providers

Now all the decoration provider code is in one place, decor_providers no
longer need to be global.

src/nvim/decoration_provider.c

@@ -9,6 +9,13 @@
 #include "nvim/highlight.h"
 #include "nvim/lua/executor.h"
 
+static kvec_t(DecorProvider) decor_providers = KV_INITIAL_VALUE;
+
+#define DECORATION_PROVIDER_INIT(ns_id) (DecorProvider) \
+  { ns_id, false, LUA_NOREF, LUA_NOREF, \
+    LUA_NOREF, LUA_NOREF, LUA_NOREF, \
+    LUA_NOREF, -1 }
+
 static bool decor_provider_invoke(NS ns_id, const char *name, LuaRef ref,
                                   Array args, bool default_true, char **perr)
 {