Add config for gemini code assist in the repository (#8607)

This follows
https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github
for adding some settings before we enable code assist here.

The style guide text is copied from the documentation, as I don't think
we have any codified Java conventions at the moment. We can add more to
this file in the future though, as it is meant to be human-readable
instructions.

Author: helin24

.gemini/config.yaml

@@ -0,0 +1,13 @@
+# These are all defaults for now. Descriptions for these settings: https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#config.yaml-schema
+have_fun: false
+code_review:
+  disable: false
+  comment_severity_threshold: MEDIUM
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: true
+    code_review: true
+    include_drafts: true
+ignore_patterns: []
+

.gemini/styleguide.md

@@ -0,0 +1,9 @@
+# Flutter Plugin for IntelliJ Style Guide
+
+## Key Principles
+* **Readability:** Code should be easy to understand for all team members.
+* **Maintainability:** Code should be easy to modify and extend.
+* **Consistency:** Adhering to a consistent style across all projects improves
+  collaboration and reduces errors.
+* **Performance:** While readability is paramount, code should be efficient.
+