Add issue forms (#204)

Signed-off-by: Byron Ruth <byron@nats.io>

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Discussion
+    url: https://github.com/nats-io/nats-server/discussions
+    about: Ideal for ideas, feedback, or longer form questions.
+  - name: Chat
+    url: https://slack.nats.io
+    about: Ideal for short, one-off questions, general conversation, and meeting other NATS users!

.github/ISSUE_TEMPLATE/defect.yml

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+---
+name: Defect
+description: Report a defect, such as a bug or regression.
+labels:
+  - defect
+body:
+  - type: textarea
+    id: versions
+    attributes:
+      label: What version were you using?
+      description: Include the server version (`nats-server --version`) and any client versions when observing the issue.
+    validations:
+      required: true
+  - type: textarea
+    id: environment
+    attributes:
+      label: What environment was the server running in?
+      description: This pertains to the operating system, CPU architecture, and/or Docker image that was used.
+    validations:
+      required: true
+  - type: textarea
+    id: steps
+    attributes:
+      label: Is this defect reproducible?
+      description: Provide best-effort steps to showcase the defect.
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Given the capability you are leveraging, describe your expectation?
+      description: This may be the expected behavior or performance characteristics.
+    validations:
+      required: true
+  - type: textarea
+    id: actual
+    attributes:
+      label: Given the expectation, what is the defect you are observing?
+      description: This may be an unexpected behavior or regression in performance.
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/proposal.yml

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+---
+name: Proposal
+description: Propose an enhancement or new feature.
+labels:
+  - proposal
+body:
+  - type: textarea
+    id: usecase
+    attributes:
+      label: What motivated this proposal?
+      description: Describe the use case justifying this request.
+    validations:
+      required: true
+  - type: textarea
+    id: change
+    attributes:
+      label: What is the proposed change?
+      description: This could be a behavior change, enhanced API, or a branch new feature.
+    validations:
+      required: true
+  - type: textarea
+    id: benefits
+    attributes:
+      label: Who benefits from this change?
+      description: Describe how this not only benefits you.
+    validations:
+      required: false
+  - type: textarea
+    id: alternates
+    attributes:
+      label: What alternatives have you evaluated?
+      description: This could be using existing features or relying on an external dependency.
+    validations:
+      required: false