Automatically indent the cursor when adding a newline after a { in Gleam code files (#8097)

Fixes: https://github.com/zed-industries/zed/issues/7295

Release Notes:

- Fixed a bug where adding a newline after a `{` would not automatically
indent the cursor in Gleam code files
([#7295](https://github.com/zed-industries/zed/issues/7295)).

assets/settings/default.json

@@ -494,6 +494,9 @@
     "Elixir": {
       "tab_size": 2
     },
+    "Gleam": {
+      "tab_size": 2
+    },
     "Go": {
       "tab_size": 4,
       "hard_tabs": true,

crates/zed/src/languages/gleam/indents.scm

@@ -0,0 +1 @@
+(_ "{" "}" @end) @indent