ci: modernize GitHub workflows and templates

Update CI workflow: restrict triggers to master+PRs, add concurrency
groups and npm cache. Update release-please and stale workflows to
latest action versions. Replace single ISSUE_TEMPLATE.md with YAML-
based templates (bug report, feature request, config). Rewrite
CONTRIBUTING.md for awesome-cordova-plugins.

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,35 @@
+name: Feature Request
+description: Suggest a new plugin wrapper or enhancement
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Have a plugin you'd like to see supported? Since Awesome Cordova Plugins
+        is a set of wrappers for Cordova plugins, adding support is as easy as
+        creating a new wrapper.
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: What plugin or feature would you like to see added or improved?
+    validations:
+      required: true
+
+  - type: input
+    id: plugin-repo
+    attributes:
+      label: Cordova plugin repository
+      description: Link to the Cordova plugin (if requesting a new wrapper).
+      placeholder: https://github.com/example/cordova-plugin-example
+    validations:
+      required: false
+
+  - type: textarea
+    id: use-case
+    attributes:
+      label: Use case
+      description: Describe why this would be useful.
+    validations:
+      required: false