api: Rename dict_set_value to dict_set_var

Reasonings:
1. It is not used for anything, but scope dictionaries currenly. So there is no 
   need to generalize and split it into dict_set_var (which will contain some 
   scope-dictionary-specific checks) and dict_set_value (which will work for any 
   dictionary).
2. Check for key size is no longer valid for non-scope dictionaries: you *can* 
   use empty keys there. In scope dictionaries also, but you actually are not 
   supposed to store there anything, but variables.

Note that actually one may still do

    let b:[''] = 1

and “bypass” check for variable name. It won’t change what `echo b:` will show, 
but it may affect code which iterates over scope dictionary keys and sets them 
to something (if there is such code).

src/nvim/terminal.c

@@ -623,12 +623,12 @@ static void buf_set_term_title(buf_T *buf, char *title)
     FUNC_ATTR_NONNULL_ALL
 {
   Error err;
-  dict_set_value(buf->b_vars,
-                 cstr_as_string("term_title"),
-                 STRING_OBJ(cstr_as_string(title)),
-                 false,
-                 false,
-                 &err);
+  dict_set_var(buf->b_vars,
+               STATIC_CSTR_AS_STRING("term_title"),
+               STRING_OBJ(cstr_as_string(title)),
+               false,
+               false,
+               &err);
 }
 
 static int term_settermprop(VTermProp prop, VTermValue *val, void *data)