build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+---
+
+<!--
+Feel free to remove paragraphs that don't apply to your request.
+-->
+
+### Description
+
+<!--
+Describe the feature / a possible solution you'd like to see.
+-->
+
+### Considered alternatives
+
+<!--
+A clear and concise description of any alternative solutions or features you've considered.
+-->
+
+### Additional context
+
+<!--
+Add any context such as screenshots, related issues or whatever you consider relevant here.
+-->