Add stub of workflow for checking the state of upstream project (#261)

Part of #260.

Adding such simple workflow to be able to run it through the UI, and
further iterate on it.

.github/workflows/check-upstream.main.kts

@@ -0,0 +1,36 @@
+#!/usr/bin/env kotlin
+@file:Repository("https://repo1.maven.org/maven2/")
+@file:DependsOn("io.github.typesafegithub:github-workflows-kt:3.0.1")
+
+@file:Repository("https://bindings.krzeminski.it/")
+@file:DependsOn("actions:checkout:v4")
+
+import io.github.typesafegithub.workflows.actions.actions.Checkout
+import io.github.typesafegithub.workflows.domain.RunnerType
+import io.github.typesafegithub.workflows.domain.triggers.WorkflowDispatch
+import io.github.typesafegithub.workflows.dsl.workflow
+
+workflow(
+    name = "Build",
+    on = listOf(
+        WorkflowDispatch(),
+    ),
+    sourceFile = __FILE__,
+) {
+    job(
+        id = "check",
+        runsOn = RunnerType.UbuntuLatest,
+    ) {
+        uses(action = Checkout())
+        run(
+            name = "Clone snakeyaml-engine",
+            command = "git clone https://bitbucket.org/snakeyaml/snakeyaml-engine.git && cd snakeyaml-engine"
+        )
+        run(
+            name = "Calculate changes between the last synced change and current state",
+            // TODO: store this commit hash in a file
+            // TODO: get the numerical value (| wc -l)
+            command = "git log --oneline ef7ebe9c06e963e13f4ab465f300a0dd6f0940c9..master"
+        )
+    }
+}

.github/workflows/check-upstream.yaml

@@ -0,0 +1,34 @@
+# This file was generated using Kotlin DSL (.github/workflows/check-upstream.main.kts).
+# If you want to modify the workflow, please change the Kotlin file and regenerate this YAML file.
+# Generated with https://github.com/typesafegithub/github-workflows-kt
+
+name: 'Build'
+on:
+  workflow_dispatch: {}
+jobs:
+  check_yaml_consistency:
+    name: 'Check YAML consistency'
+    runs-on: 'ubuntu-latest'
+    steps:
+    - id: 'step-0'
+      name: 'Check out'
+      uses: 'actions/checkout@v4'
+    - id: 'step-1'
+      name: 'Execute script'
+      run: 'rm ''.github/workflows/check-upstream.yaml'' && ''.github/workflows/check-upstream.main.kts'''
+    - id: 'step-2'
+      name: 'Consistency check'
+      run: 'git diff --exit-code ''.github/workflows/check-upstream.yaml'''
+  check:
+    runs-on: 'ubuntu-latest'
+    needs:
+    - 'check_yaml_consistency'
+    steps:
+    - id: 'step-0'
+      uses: 'actions/checkout@v4'
+    - id: 'step-1'
+      name: 'Clone snakeyaml-engine'
+      run: 'git clone https://bitbucket.org/snakeyaml/snakeyaml-engine.git && cd snakeyaml-engine'
+    - id: 'step-2'
+      name: 'Calculate changes between the last synced change and current state'
+      run: 'git log --oneline ef7ebe9c06e963e13f4ab465f300a0dd6f0940c9..master'