Add GitHub issue templates

Author: hukkin

.github/ISSUE_TEMPLATE/backslash_escape.yaml

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+contact_links:
+  - name: Why does Mdformat use backslash to escape my Markdown engine specific syntax? ⁉️
+    url: https://github.com/executablebooks/mdformat?tab=readme-ov-file#why-does-mdformat-backslash-escape-special-syntax-specific-to-mkdocs--hugo--obsidian--github--some-other-markdown-engine
+    about: It's likely you need to install a plugin. See the FAQ more info.

.github/ISSUE_TEMPLATE/bug.yaml

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+name: Report a bug 🐛
+description: Mdformat crashes or exit code is non-zero, output is invalid CommonMark, rendered document is visually different after formatting...
+labels: [bug]
+body:
+- type: textarea
+  id: describe
+  attributes:
+    label: Describe the bug
+    description: |
+      Provide a short description (one or two sentences) about the problem. What did you expect to happen, and what is actually happening?
+    value: |
+      **context**
+      When I do ___.
+      
+      **expectation**
+      I expected ___ to occur.
+      
+      **bug**
+      But instead ___ happens
+      Here's an error message I ran into...
+
+      ```console
+      $ mdformat README.md
+      ERROR ...
+      ```
+
+  validations:
+    required: true
+
+- type: textarea
+  id: reproduce
+  attributes:
+    label: Reproduce the bug
+    description: |
+      Provide information that others may use to re-produce this behavior.
+      For example:
+      
+      - A self-contained command to reproduce the error, e.g.
+        ```console
+        printf -- '- item A\n  - item A.a\n- item B' | pipx run mdformat -
+        ```
+      - Minimal content of a file that when formatted causes the error. Command to format the file.
+
+    placeholder: |
+      1. Create a file test.md with the content '...'
+      2. Run `mdformat test.md`
+      3. See error
+  validations:
+    required: true
+
+- type: textarea
+  id: environment
+  attributes:
+    label: List your environment
+    description: |
+      List the environment needed to reproduce the error.
+      Here are a few ideas:
+
+      - The output of:      
+        ```console
+        mdformat --version
+        ```
+      - The version of Python you're using.
+      - Your operating system
+    placeholder: |
+      ```
+      ❯ mdformat --version
+        mdformat 0.7.18 (mdformat_black: 0.1.1)
+      ```
+  validations:
+    required: true

.github/ISSUE_TEMPLATE/enhancement.yaml

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+name: Request an enhancement 💡
+description: Suggest an idea for this project
+labels: [enhancement]
+body:
+- type: textarea
+  id: context
+  attributes:
+    label: Context
+    description: |
+      - Provide background to help others understand this issue.
+      - Describe the problem or need you'd like to address.
+  validations:
+    required: true
+
+
+- type: textarea
+  id: proposal
+  attributes:
+    label: Proposal
+    description: |
+      - A simple and clear description of what you're proposing.
+      - Ideas or constraints for how to implement this proposal
+      - Important considerations to think about or discuss
+  validations:
+    required: false