Address security alert: Update RCMD check workflow (#29)

Closes #27
Thanks @Ale0x78

.github/.gitignore

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+*.html

.github/workflows/check-full.yaml

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+# Workflow derived from https://github.com/r-lib/actions/tree/master/examples
+# Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
+#
+# NOTE: This workflow is overkill for most R packages and
+# check-standard.yaml is likely a better choice.
+# usethis::use_github_action("check-standard") will install it.
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+name: R-CMD-check
+
+jobs:
+  R-CMD-check:
+    runs-on: ${{ matrix.config.os }}
+
+    name: ${{ matrix.config.os }} (${{ matrix.config.r }})
+
+    strategy:
+      fail-fast: false
+      matrix:
+        config:
+          - {os: macOS-latest,   r: 'release'}
+
+          - {os: windows-latest, r: 'release'}
+          # Use 3.6 to trigger usage of RTools35
+          - {os: windows-latest, r: '3.6'}
+
+          # Use older ubuntu to maximise backward compatibility
+          - {os: ubuntu-18.04,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-18.04,   r: 'release'}
+          - {os: ubuntu-18.04,   r: 'oldrel-1'}
+          - {os: ubuntu-18.04,   r: 'oldrel-2'}
+          - {os: ubuntu-18.04,   r: 'oldrel-3'}
+          - {os: ubuntu-18.04,   r: 'oldrel-4'}
+
+    env:
+      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+      R_KEEP_PKG_SOURCE: yes
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: r-lib/actions/setup-pandoc@v1
+
+      - uses: r-lib/actions/setup-r@v1
+        with:
+          r-version: ${{ matrix.config.r }}
+          http-user-agent: ${{ matrix.config.http-user-agent }}
+          use-public-rspm: true
+
+      - uses: r-lib/actions/setup-r-dependencies@v1
+        with:
+          extra-packages: rcmdcheck
+
+      - uses: r-lib/actions/check-r-package@v1