chore: add support for index expressions in formatter

tooling/nargo_fmt/src/visitor/expr.rs

@@ -39,6 +39,12 @@ impl FmtVisitor<'_> {
                     self.format_expr(infix.rhs)
                 )
             }
+            ExpressionKind::Index(index_expr) => {
+                let formatted_collection =
+                    self.format_expr(index_expr.collection).trim_end().to_string();
+                let formatted_index = self.format_expr(index_expr.index);
+                format!("{}[{}]", formatted_collection, formatted_index)
+            }
             ExpressionKind::Literal(literal) => match literal {
                 Literal::Integer(_) => slice!(self, span.start(), span.end()).to_string(),
                 Literal::Array(ArrayLiteral::Repeated { repeated_element, length }) => {

tooling/nargo_fmt/tests/expected/index.nr

@@ -0,0 +1,5 @@
+fn foo() {
+    let arr = [10, 20, 30, 40];
+    arr[2];
+    arr[2];
+}

tooling/nargo_fmt/tests/input/index.nr

@@ -0,0 +1,5 @@
+fn foo() {
+    let arr = [10, 20, 30, 40];
+    arr [2];
+    arr   [2];
+}