feat(ftplugin): change 'commentstring' to // %s for C/C++ (#29085)

Problem: The default commentstring for C/C++ can lead to invalid code 
when commenting and does not match the Nvim codebase.

Solution: Change commentstring to `// %s` as used by Neovim. Also
set all commentstrings that derive from the default C string explicitly
(and correctly).

runtime/ftplugin/arduino.lua

@@ -0,0 +1 @@
+vim.bo.commentstring = '// %s'

runtime/ftplugin/c.lua

@@ -1,5 +1,5 @@
 -- These are the default option values in Vim, but not in Nvim, so must be set explicitly.
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '// %s'
 vim.bo.define = '^\\s*#\\s*define'
 vim.bo.include = '^\\s*#\\s*include'
 

runtime/ftplugin/calender.lua

@@ -1 +1 @@
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '/* %s */'

runtime/ftplugin/ch.lua

@@ -0,0 +1 @@
+vim.bo.commentstring = '// %s'

runtime/ftplugin/cs.lua

@@ -1 +1 @@
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '// %s'

runtime/ftplugin/css.lua

@@ -1 +1 @@
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '/* %s */'

runtime/ftplugin/d.lua

@@ -1 +1 @@
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '// %s'

runtime/ftplugin/indent.lua

@@ -1 +1 @@
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '// %s'

runtime/ftplugin/objc.lua

@@ -0,0 +1 @@
+vim.bo.commentstring = '// %s'

runtime/ftplugin/xdefaults.lua

@@ -1 +1 @@
-vim.bo.commentstring = '/*%s*/'
+vim.bo.commentstring = '! %s'

runtime/ftplugin/xs.lua

@@ -0,0 +1 @@
+vim.bo.commentstring = '// %s'

test/functional/lua/runtime_spec.lua

@@ -406,7 +406,7 @@ describe('runtime:', function()
     eq('', eval('&commentstring'))
     eq('', eval('&omnifunc'))
     exec('edit file.cpp')
-    eq('/*%s*/', eval('&commentstring'))
+    eq('// %s', eval('&commentstring'))
     eq('ccomplete#Complete', eval('&omnifunc'))
   end)
 end)