Add a form GitHub issue template for proposals

GitHub now supports forms as issue templates (currently in private beta).
Godot has been opted into this private beta, which makes this new feature
usable on the Godot proposals repository.

Inspired by the ImageMagick form:

https://github.com/ImageMagick/ImageMagick/blob/main/.github/ISSUE_TEMPLATE/bug-report.yml

.github/ISSUE_TEMPLATE/feature---enhancement-proposal.md

@@ -1,25 +0,0 @@
----
-name: Feature / Enhancement proposal
-about: 'Open a new proposal'
-title: ''
-labels: ''
-assignees: ''
-
----
-
-<!--
-Please fill in *all* the questions below and don't remove any of them.
-Proposals not following the template below will be closed immediately.
--->
-
-### Describe the project you are working on
-
-### Describe the problem or limitation you are having in your project
-
-### Describe the feature / enhancement and how it helps to overcome the problem or limitation
-
-### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams
-
-### If this enhancement will not be used often, can it be worked around with a few lines of script?
-
-### Is there a reason why this should be core and not an add-on in the asset library?

.github/ISSUE_TEMPLATE/feature_enhancement_proposal.yml

@@ -0,0 +1,52 @@
+name: Feature / Enhancement proposal
+description: Open a new proposal
+body:
+
+- type: markdown
+  attributes:
+    value: |
+      - Write a descriptive proposal title above.
+      - Search [open](https://github.com/godotengine/godot-proposals/issues) and [closed](https://github.com/godotengine/godot-proposals/issues?q=is%3Aissue+is%3Aclosed) proposals to ensure the feature has not already been suggested.
+
+- type: textarea
+  attributes:
+    label: Describe the project you are working on
+    description: This is important to know the *context* in which the feature is being proposed. Features used in real-world projects are more likely to be added.
+    placeholder: Example - "A 3D space game with heavy use of cut-out animation"
+  validations:
+    required: true
+
+- type: textarea
+  attributes:
+    label: Describe the problem or limitation you are having in your project
+    placeholder: Example - "The 3D selection tools are cumbersome to use."
+  validations:
+    required: true
+
+- type: textarea
+  attributes:
+    label: Describe the feature / enhancement and how it helps to overcome the problem or limitation
+    placeholder: Example - "Add keyboard shortcuts to select groups of nodes quickly."
+  validations:
+    required: true
+
+- type: textarea
+  attributes:
+    label: Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams
+    placeholder: Example - "When the user presses Ctrl + Shift + G, select all nodes that have the group..."
+  validations:
+    required: true
+
+- type: textarea
+  attributes:
+    label: If this enhancement will not be used often, can it be worked around with a few lines of script?
+    placeholder: Example - "This can be worked around with an editor plugin, but it is not particularly convenient."
+  validations:
+    required: true
+
+- type: textarea
+  attributes:
+    label: Is there a reason why this should be core and not an add-on in the asset library?
+    placeholder: Example - "This is about improving the 3D editor usability out of the box."
+  validations:
+    required: true