deploy: autorelease on tags

[casperdcl]

Fixes #845

• auto-draft release notes on tag push
  • facilitates adding release assets easily in future (for Standalone pkg #842)
• tidy checkbot
• add tagbot
• Release method (to be updated in https://cml.dev/doc/contributing/core post-merge):

  - `git checkout master && git pull && git checkout -b M.m.p && npm version M.m.p && git push && gh pr create`
  - Merge the resulting PR
- - Draft a new [release](https://github.com/iterative/cml/releases), ensuring
-   all commits since last release are summarised in the release notes
+ - `git checkout master && git pull && git tag -f vM.m.p && git push --tags`
+    or comment `/tag vM.m.p SHA` in the PR
+ - Wait for a draft to appear under [releases](https://github.com/iterative/cml/releases)
+ - Check & publish the draft

[casperdcl]

we can ignore the failing windows tests since they'll be fixed in #842

[DavidGOrtega]

👍

Awesome @casperdcl

[0x2b3bfa0]

🤷🏼‍♂️ If you prefer this release method, LGTM.

[casperdcl]

don't know yet; we'll see how it goes 🥼