parser: make to and table independently optional in add column

Previously the grammar accepted only `to table` together or
neither. The user-stated convention is that bare table
identifiers are accepted in unambiguous positions (matching
how `add 1:n relationship from <T>.<col> to <T>.<col>` takes
bare table names). Both `to` and `table` are now or_not'd
independently, so all four combinations parse identically.

Updates the in-app `help` listing to advertise the new
shape: `add column [to] [table] <T>: <col> (<type>)`.

3 new parser tests cover the variants.

src/app.rs

@@ -1108,7 +1108,7 @@ impl App {
             "DSL data commands (in simple mode):",
             "  create table <T> with pk [<col>:<type>...]",
             "  drop table <T>",
-            "  add column [to table] <T>: <col> (<type>)",
+            "  add column [to] [table] <T>: <col> (<type>)",
             "  add 1:n relationship [as <name>] from <P>.<col> to <C>.<col>",
             "    [on delete <action>] [on update <action>] [--create-fk]",
             "  drop relationship <name>",