Implement date-checker

This tool looks for HTML comments like `<!-- date: 2021-01 -->` in each
Markdown source file and compiles a list of dates that are older than
six months. It then opens an issue with that list, with checkboxes for
each file and date. Note that it will only open an issue if there was at
least one date older than six months; it does nothing if the list is
empty.

This tool is automatically run monthly in a GitHub Actions workflow.
I have tested the tool on a private repo and confirmed that it works.

Author: camelid

Diff for: .github/workflows/date-check.yml

@@ -0,0 +1,44 @@
+name: Date-Check
+
+on:
+  schedule:
+    # Run at noon UTC every 1st of the month
+    - cron: '00 12 01 * *'
+
+  # Allow manually starting the workflow
+  workflow_dispatch:
+
+jobs:
+  date-check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v2
+
+      - name: Run `date-check`
+        working-directory: ci/date-check
+        run: |
+          cargo run -- ../../src/ > ../../date-check-output.txt
+
+      - name: Open issue
+        uses: actions/github-script@v3
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const fs = require('fs');
+
+            const rawText = fs.readFileSync('date-check-output.txt', { encoding: 'utf8' });
+            const title = rawText.split('\n')[0];
+            if (title != 'empty') {
+                const body = rawText.split('\n').slice(1).join('\n');
+                github.issues.create({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  title,
+                  body,
+                });
+                console.log('Opened issue.');
+            } else {
+                console.log('No dates to triage.');
+            }

Diff for: .gitignore

@@ -1,4 +1,6 @@
 book
 
-# prevent accidentally changes
+# prevent accidental changes
 book.toml
+
+ci/date-check/target/

Diff for: ci/date-check/Cargo.lock

@@ -0,0 +1,12 @@
+[package]
+name = "date-check"
+version = "0.1.0"
+authors = ["Camelid <camelidcamel@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+glob = "0.3"
+regex = "1"
+chrono = "0.4"