chore: Add GitHub issue templates.

.github/ISSUE_TEMPLATE/bug-report.yaml

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+name: "Bug Report"
+description: "Report software deficiencies"
+labels: ["bug"]
+body:
+  - type: "markdown"
+    attributes:
+      value: |
+        Use this form to report any functional or performance bugs you've found in the software.
+
+        Be sure to check if your [issue](https://github.com/y-scope/ystdlib-cpp/issues) has already
+        been reported.
+
+  - type: "textarea"
+    attributes:
+      label: "Bug"
+      description: "Describe what's wrong and if applicable, what you expected instead."
+    validations:
+      required: true
+
+  - type: "input"
+    attributes:
+      label: "ystdlib-cpp version"
+      description: "The release version number or development commit hash that has the bug."
+      placeholder: "Version number or commit hash"
+    validations:
+      required: true
+
+  - type: "textarea"
+    attributes:
+      label: "Environment"
+      description: "The environment in which you're running ystdlib-cpp."
+      placeholder: "OS version, Docker version, etc."
+    validations:
+      required: true
+
+  - type: "textarea"
+    attributes:
+      label: "Reproduction steps"
+      description: "List each step required to reproduce the bug."
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yaml

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+blank_issues_enabled: true

.github/ISSUE_TEMPLATE/feature-request.yaml

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+name: "Feature/Change Request"
+description: "Request a feature or change"
+labels: ["enhancement"]
+body:
+  - type: "markdown"
+    attributes:
+      value: |
+        Use this form to request a feature/change in the software, or the project as a whole.
+  - type: "textarea"
+    attributes:
+      label: "Request"
+      description: "Describe your request and why it's important."
+    validations:
+      required: true
+
+  - type: "textarea"
+    attributes:
+      label: "Possible implementation"
+      description: "Describe any implementations you have in mind."
+    validations:
+      required: true