Modernize spell check CI workflow

The previous version of the workflow was using an experimental spell check GitHub Actions action that we have decided to
abandon in favor of the official codespell action.

Author: per1234

.codespellrc

@@ -0,0 +1,7 @@
+# See: https://github.com/codespell-project/codespell#using-a-config-file
+[codespell]
+# In the event of a false positive, add the problematic word, in all lowercase, to a comma-separated list here:
+ignore-words-list = ,
+check-filenames =
+check-hidden =
+skip = ./.git

.github/workflows/spell-check.yml

@@ -1,11 +1,22 @@
 name: Spell Check
-on: [push, pull_request]
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+  pull_request:
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch new misspelling detections resulting from dictionary updates.
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
 jobs:
- build:
-   runs-on: ubuntu-latest
-
-   steps:
-     - uses: actions/checkout@v1
-       with:
-         fetch-depth: 1
-     - uses: arduino/actions/libraries/spell-check@master
+  spellcheck:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Spell check
+        uses: codespell-project/actions-codespell@master

README.adoc

@@ -4,6 +4,7 @@
 = {repository-name} Library for Arduino =
 
 image:https://github.com/{repository-owner}/{repository-name}/actions/workflows/compile-examples.yml/badge.svg["Compile Examples status", link="https://github.com/{repository-owner}/{repository-name}/actions/workflows/compile-examples.yml"]
+image:https://github.com/{repository-owner}/{repository-name}/actions/workflows/spell-check.yml/badge.svg["Spell Check status", link="https://github.com/{repository-owner}/{repository-name}/actions/workflows/spell-check.yml"]
 
 Allows you to read the temperature sensors connected to your MKR THERM shield.