chore: add issue templates (#521)

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: 🐛 Bug Report
+description: Create a bug report to help us improve Avante
+title: 'bug: '
+labels: ['bug']
+body:
+  - type: markdown
+    id: issue-already-exists
+    attributes:
+      value: |
+        Please search to see if an issue already exists for the bug you encountered.
+        See [Searching Issues and Pull Requests](https://docs.github.com/en/search-github/searching-on-github/searching-issues-and-pull-requests) for how to use the GitHub search bar and filters.
+  - type: textarea
+    id: describe-the-bug
+    validations:
+      required: true
+    attributes:
+      label: Describe the bug
+      description: Please provide a clear and concise description about the problem you ran into.
+      placeholder: This happened when I...
+  - type: textarea
+    id: to-reproduce
+    validations:
+      required: false
+    attributes:
+      label: To reproduce
+      description: |
+        Please provide a code sample or a code snipet to reproduce said problem. If you have code snippets, error messages, stack trace please also provide them here.
+
+        **IMPORTANT**: make sure to use [code tag](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks#syntax-highlighting) to correctly format your code. Screenshot is helpful but don't use it for code snippets as it doesn't allow others to copy-and-paste your code.
+
+        Provide your neovim version with `neovim -v`
+      placeholder: |
+        Give a minimal config to reproduce the issue.k
+  - type: textarea
+    id: expected-behavior
+    validations:
+      required: false
+    attributes:
+      label: Expected behavior
+      description: 'A clear and concise description of what you would expect to happen.'
+  - type: textarea
+    id: environment-info
+    attributes:
+      label: Environment
+      description: |
+        Please share your environment with us.
+      placeholder: |
+        neovim version: ...
+        distribution (if any): ...
+        platform: ...
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+version: 2.1

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: 🚀 Feature Request
+description: Submit a proposal/request for new Avante feature.
+title: 'feature: '
+labels: ['new-feature', 'enhancement']
+body:
+  - type: textarea
+    id: feature-request
+    validations:
+      required: true
+    attributes:
+      label: Feature request
+      description: |
+        A clear and concise description of the feature request.
+      placeholder: |
+        I would like it if...
+  - type: textarea
+    id: motivation
+    validations:
+      required: false
+    attributes:
+      label: Motivation
+      description: |
+        Please outline the motivation for this feature request. Is your feature request related to a problem? e.g., I'm always frustrated when [...].
+        If this is related to another issue, please link here too.
+        If you have a current workaround, please also provide it here.
+      placeholder: |
+        This feature would solve ...
+  - type: textarea
+    id: other
+    attributes:
+      label: Other
+      description: |
+        Is there any way that you could help, e.g. by submitting a PR?
+      placeholder: |
+        I would love to contribute ...