chore: Convert the feature request template to an issue form (#3036)

Similar to: https://github.com/flame-engine/flame/pull/3027

This PR is doing:
- Adding a new issue form to replace the template found here:
https://github.com/flame-engine/flame/blob/main/.github/ISSUE_TEMPLATE/2_feature.md
- Also _deletes_ the template, so we don't see duplicates when a user
comes to create a feature request.

Proposed new form looks like:
<img width="957" alt="image"
src="https://github.com/flame-engine/flame/assets/13091188/86dcbe60-d0fb-4f77-ab97-8895e3b69e8f">

I took some leniences in design decisions with making, which happy to
change if needed:
- The problem _and_ proposal text areas **required**
- Turned the "are you interested in doing a PR?" into a checkbox.

.github/ISSUE_TEMPLATE/2_feature.md

@@ -1,21 +0,0 @@
----
-name: Feature request.
-about: Suggest a new feature for Flame.
-title: ''
-labels: 'enhancement'
-assignees: ''
-
----
-
-<!-- When reporting a feature, please read this complete template and fill all the questions in order to get a better response -->
-
-# Problem to solve
-<!-- Which problem would be solved with this feature? -->
-
-# Proposal
-<!-- What do you propose as a solution? Add as much information as you can! -->
-
-# More information
-<!-- Do you have any other useful information about this feature report? Please write it down here -->
-<!-- Possible helpful information: references to other sites/repositories -->
-<!-- Are you interested in working on a PR for this? -->

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,33 @@
+name: Feature request
+description: Suggest a new feature for Flame.
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        When suggesting a feature, please read this complete form and fill in all the questions in order to get a better response!
+  - type: textarea
+    id: problem-to-solve
+    attributes:
+      label: Problem to solve
+      description: Which problem would be solved with this feature?
+    validations:
+      required: true
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposal
+      description: What do you propose as a solution? Add as much information as you can!
+    validations:
+      required: true
+  - type: textarea
+    id: more-information
+    attributes:
+      label: More information
+      description: Do you have any other useful information about this feature report? Please write it down here. Possible helpful information are references to other sites/repositories.
+  - type: checkboxes
+    id: other
+    attributes:
+      label: Other
+      options:
+        - label: Are you interested in working on a PR for this?