Allow lua REPL to warn about using local variables (#367)

`local varname = value` results in `varname` being inaccessible in
the next REPL input. This is often unintended and can lead to confusing
behaviour. We produce a warning when this occurs.

src/main/resources/assets/computercraft/lua/bios.lua

@@ -932,6 +932,11 @@ settings.define("motd.path", {
     description = [[The path to load random messages from. Should be a colon (":") separated string of file paths.]],
     type = "string",
 })
+settings.define("lua.warn_against_use_of_local", {
+    default = true,
+    description = [[Print a message when input in the Lua REPL starts with the word 'local'. Local variables defined in the Lua REPL are be inaccessable on the next input.]],
+    type = "boolean",
+})
 if term.isColour() then
     settings.define("bios.use_multishell", {
         default = true,