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/dsl/parser.rs

@@ -129,14 +129,19 @@ fn command_parser<'a>()
         .ignore_then(identifier())
         .map(|name| Command::DropTable { name });
 
-    // `to table` is optional — both `add column to table T: c (text)`
-    // and `add column T: c (text)` parse identically.
-    let to_table_optional = keyword_ci("to")
-        .ignore_then(keyword_ci("table"))
-        .or_not();
+    // Both `to` and `table` are independently optional —
+    // `add column to table T: c (text)`,
+    // `add column to T: c (text)`,
+    // `add column table T: c (text)`,
+    // and `add column T: c (text)` all parse identically.
+    // Matches the convention elsewhere in the DSL where bare
+    // identifiers are accepted in unambiguous positions.
+    let optional_to = keyword_ci("to").or_not();
+    let optional_table = keyword_ci("table").or_not();
     let add_column = keyword_ci("add")
         .ignore_then(keyword_ci("column"))
-        .ignore_then(to_table_optional)
+        .ignore_then(optional_to)
+        .ignore_then(optional_table)
         .ignore_then(identifier())
         .then_ignore(just(':').padded())
         .then(identifier())
@@ -764,6 +769,46 @@ mod tests {
         }
     }
 
+    #[test]
+    fn add_column_accepts_bare_table_name() {
+        // `to table` are both optional; bare table identifier
+        // is accepted in this unambiguous position.
+        assert_eq!(
+            ok("add column Customers: Name (text)"),
+            Command::AddColumn {
+                table: "Customers".to_string(),
+                column: "Name".to_string(),
+                ty: Type::Text,
+            }
+        );
+    }
+
+    #[test]
+    fn add_column_accepts_to_alone() {
+        // `to` without `table`.
+        assert_eq!(
+            ok("add column to Customers: Name (text)"),
+            Command::AddColumn {
+                table: "Customers".to_string(),
+                column: "Name".to_string(),
+                ty: Type::Text,
+            }
+        );
+    }
+
+    #[test]
+    fn add_column_accepts_table_alone() {
+        // `table` without `to`.
+        assert_eq!(
+            ok("add column table Customers: Name (text)"),
+            Command::AddColumn {
+                table: "Customers".to_string(),
+                column: "Name".to_string(),
+                ty: Type::Text,
+            }
+        );
+    }
+
     #[test]
     fn add_column_tolerates_whitespace_around_punctuation() {
         assert_eq!(