ci: add pr links automatically

[yihau]

Motivation

a improvement for https://github.com/solana-labs/move/issues/51. try to make the process become automatic. this new action will be triggered when someone create a PR. it will

1. commit a empty change with the PR number.
2. move the commit as the first commit of the PR. (via rebase)
3. push -f

if we don't like it, we can change this process to either

• only append the pr message to the last commit of a PR.
• the same process as this one but happen in the last commit instead of the head commit
• other possibilities ...
  Use Cases

When a PR has been created

When the action successes

the demo PR: https://github.com/yihau/solana-move/pull/27