Add script to simplify release preparation (#218)

[Demo](https://screencast.googleplex.com/cast/NjM2MTM3MDEwOTkzNTYxNnw3OWJhMzlmMi1lYg)

If this looks good, I can update our release documentation as well

tools/release/prepare-release.sh

@@ -0,0 +1,73 @@
+#!/bin/bash
+
+readonly VERSION_REGEX='__version__ = "([0-9.]+)"' # Regex to find the version string
+readonly CHANGELOG_FILE="CHANGELOG.md"
+readonly INIT_FILE="src/kagglehub/__init__.py"
+readonly FEATURE_BRANCH_PREFIX="release-"
+# Determine the root repo directory based on the location of this script
+readonly ROOT_REPO_DIR=$(dirname $(dirname $(dirname $(realpath $0))))
+
+# New version specified by the person preparing the release
+NEW_VERSION=$1
+
+# Exit if any command fails
+set -e
+
+# Run all the commands from repo root
+cd "$ROOT_REPO_DIR"
+
+# 1. Pull latest main
+git checkout main
+git pull origin main
+
+# 2. Fetch latest tag
+git fetch --tags
+
+# 3. Determine commit hash of latest tag
+latest_tag=$(git describe --tags --abbrev=0) # Gets the latest tag name
+tag_hash=$(git rev-list -n 1 "$latest_tag")
+
+# 4. Gather commits since that hash
+commits=$(git log --pretty=format:"%s" "$tag_hash..main")
+
+# 5. Update CHANGELOG.md
+new_changelog_entry="\n"
+if [[ -n "$commits" ]]; then
+    date_string=$(date +"%B %d, %Y")
+    new_changelog_entry+="## v$NEW_VERSION ($date_string)\n\n"
+
+    while IFS= read -r line; do
+        if [[ -n "$line" ]]; then
+            pr_number=$(echo "$line" | grep -o '#[0-9]*' | sed 's/#//')
+            pattern="\(#[0-9])"
+            commit_message=$(echo "$line" | sed -E 's/ \(#[0-9]*\) *//')
+            new_changelog_entry+="* ${commit_message} ([#${pr_number}](https://github.com/Kaggle/kagglehub/pull/${pr_number}))\n"
+        fi
+    done <<< "$commits"
+else
+  echo "No significant changes since last release"
+  exit 1
+fi
+
+# Read the file, separate the first line (# Changelog)
+first_line=$(head -n 1 "$CHANGELOG_FILE")
+rest_of_file=$(tail -n +2 "$CHANGELOG_FILE") # Everything after the first line
+
+# Combine: # Changelog + New Entry + Rest of the file
+echo "$first_line" > "$CHANGELOG_FILE".tmp  # Write the first line
+printf "$new_changelog_entry" >> "$CHANGELOG_FILE".tmp # Append the new entry
+echo "$rest_of_file" >> "$CHANGELOG_FILE".tmp # Append the rest of the file
+
+mv "$CHANGELOG_FILE".tmp "$CHANGELOG_FILE"
+
+# 6. Update __init__.py version
+sed -i -E "s/$VERSION_REGEX/__version__ = \"$NEW_VERSION\"/" "$INIT_FILE"
+
+# 7. Create and push feature branch
+feature_branch="$FEATURE_BRANCH_PREFIX$NEW_VERSION"
+git checkout -b "$feature_branch"
+git add "$CHANGELOG_FILE" "$INIT_FILE"
+git commit -m "Prepare release $NEW_VERSION"
+git push origin "$feature_branch"
+
+echo "Release preparation complete. Go to https://github.com/Kaggle/kagglehub/compare/$feature_branch?expand=1 to open the PR."