Add: [Actions] release workflow for both releases and nightlies

This has several ways of being triggered:
- When creating a new release via the GitHub interface. Fully
  automated that will produce new binaries, upload them, and it
  will even update the website to tell about the new version.
- When triggered in an automated way from OpenTTD/workflows to
  start a nightly.
- Manually via the Release workflow, which accepts branches,
  Pull Requests and tags to build.

Rerunning a job is safe and should be without issues. Everything
retriggers and updates what-ever might have been broken. In fact,
except for dates, it should produce identical results.

Co-authored-by: Charles Pigott <charlespigott@googlemail.com>

.github/changelog.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+tag=$(git name-rev --name-only --tags --no-undefined HEAD 2>/dev/null | sed 's@\^0$@@')
+
+# If we are a tag, show the part of the changelog till (but excluding) the last stable
+if [ -n "$tag" ]; then
+    grep='^[0-9]\+\.[0-9]\+\.[0-9]\+[^-]'
+    next=$(cat changelog.txt | grep '^[0-9]' | awk 'BEGIN { show="false" } // { if (show=="true") print $0; if ($1=="'$tag'") show="true"} ' | grep "$grep" | head -n1 | sed 's/ .*//')
+    cat changelog.txt | awk 'BEGIN { show="false" } /^[0-9]+.[0-9]+.[0-9]+/ { if ($1=="'$next'") show="false"; if ($1=="'$tag'") show="true";} // { if (show=="true") print $0 }'
+    exit 0
+fi
+
+# In all other cases, show the git log of the last 7 days
+revdate=$(git log -1 --pretty=format:"%ci")
+last_week=$(date -d "$revdate -7days" +"%Y-%m-%d %H:%M")
+git log --after="${last_week}" --pretty=fuller