Add validate-revision workflow

It can be started manually to check that certain version bumps build fine.
It will ask to input an allow-newer line and a constraint line, which it
will add to the cabal.project file. For example: the "filepath"
allow-newer line and "filepath == 1.5" constraints line will make sure
that the build plan will have `filepath` at version 1.5.

.github/workflows/validate-revision.yml

@@ -0,0 +1,20 @@
+name: Validate Revision
+
+on:
+  workflow_dispatch:
+    inputs:
+      allow-newer:
+        description: allow-newer line
+        required: true
+        type: string
+      constraints:
+        description: constraints line
+        required: true
+        type: string
+env:
+  ALLOWNEWER: ${{ github.event.inputs.allow-newer }}
+  CONSTRAINTS: ${{ github.event.inputs.constraints }}
+
+jobs:
+  validate-main:
+    uses: ./.github/workflows/validate.yml