Add CI workflow to check for commonly misspelled words

On every push, pull request, and periodically, use the codespell-project/actions-codespell action to check for commonly
misspelled words.

In the event of a false positive, the problematic word should be added, in all lowercase, to the ignore-words-list field
of ./.codespellrc. Regardless of the case of the word in the false positive, it must be in all lowercase in the ignore
list. The ignore list is comma-separated with no spaces.

Author: per1234

Diff for: .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 = hel
+check-filenames =
+check-hidden =
+skip = ./.git,./test/external

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

@@ -0,0 +1,22 @@
+name: Spell Check
+
+# 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:
+  spellcheck:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Spell check
+        uses: codespell-project/actions-codespell@master

Diff for: README.md

@@ -2,6 +2,7 @@
 
 [![Unit Tests](https://github.com/arduino/ArduinoCore-API/workflows/Unit%20Tests/badge.svg)](https://github.com/arduino/ArduinoCore-API/actions?workflow=Unit+Tests)
 [![codecov](https://codecov.io/gh/arduino/ArduinoCore-API/branch/master/graph/badge.svg)](https://codecov.io/gh/arduino/ArduinoCore-API)
+[![Spell Check status](https://github.com/arduino/ArduinoCore-API/actions/workflows/spell-check.yml/badge.svg)](https://github.com/arduino/ArduinoCore-API/actions/workflows/spell-check.yml)
 
 This repository hosts the hardware independent layer of Arduino core.