Add prepare_release.ps1 script

Fixes MessagePack-CSharp#1195

prepare_release.ps1

@@ -0,0 +1,14 @@
+# Calculate the NPM package version, assuming the version change is in a new commit.
+git commit --allow-empty -m "Dummy commit" -q
+$NpmPackageVersion = (nbgv get-version -f json | ConvertFrom-Json).NpmPackageVersion
+git reset --mixed HEAD~ -q
+
+# Stamp the version into the package.json file and commit.
+pushd $PSScriptRoot/src/MessagePack.UnityClient/Assets/Scripts/MessagePack
+npm version $NpmPackageVersion --no-git-tag-version --allow-same-version
+git add package.json
+popd
+git commit -m "Stamp unity package version as $NpmPackageVersion"
+
+# Tag the release
+nbgv tag