fix(highlight): add missing g: prefix for colors_name (#22952)

Fix #22951.
This was fixed in Vim in patch 8.2.0613.

test/functional/ex_cmds/highlight_spec.lua

@@ -3,6 +3,9 @@ local helpers = require("test.functional.helpers")(after_each)
 local eq, command = helpers.eq, helpers.command
 local clear = helpers.clear
 local eval, exc_exec = helpers.eval, helpers.exc_exec
+local exec = helpers.exec
+local funcs = helpers.funcs
+local meths = helpers.meths
 
 describe(':highlight', function()
   local screen
@@ -45,4 +48,20 @@ describe(':highlight', function()
     eq('', eval('synIDattr(hlID("NonText"), "undercurl", "gui")'))
     eq('1', eval('synIDattr(hlID("NonText"), "underline", "gui")'))
   end)
+
+  it('clear', function()
+    meths.set_var('colors_name', 'foo')
+    eq(1, funcs.exists('g:colors_name'))
+    command('hi clear')
+    eq(0, funcs.exists('g:colors_name'))
+    meths.set_var('colors_name', 'foo')
+    eq(1, funcs.exists('g:colors_name'))
+    exec([[
+      func HiClear()
+        hi clear
+      endfunc
+    ]])
+    funcs.HiClear()
+    eq(0, funcs.exists('g:colors_name'))
+  end)
 end)