Automate more of Wasmtime's release process (#3422)

* Automate more of Wasmtime's release process

This change revamps the release process for Wasmtime and intends to make
it nearly 100% automated for major release and hopefully still pretty
simple for patch releases. New workflows are introduced as part of
this commit:

* Once a month a PR is created with major version bumps
* Specifically hinted commit messages to the `main` branch will get
  tagged and pushed to the main repository.
* On tags we'll now not only build releases after running CI but
  additionally crates will be published to crates.io.

In conjunction with other changes this means that the release process
for a new major version of Wasmtime is simply merging a PR. Patch
releases will involve running some steps locally but most of the
nitty-gritty should be simply merging the PR that's generated.

* Use an anchor in a regex

.github/workflows/publish-to-cratesio.yml

@@ -0,0 +1,24 @@
+# The purpose of this workflow is to publish the wasmtime workspace of crates
+# whenever a wasmtime tag is created. This baiscally boils down to running
+# `scripts/publish.rs` at the right time.
+
+name: "Publish to crates.io"
+
+on:
+  push:
+    tags:
+    - 'v*'
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: true
+    - run: rustup update stable && rustup default stable
+    - run: |
+        rustc scripts/publish.rs
+        ./publish publish
+      env:
+        CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}