Add a time debounce property

[10hendersonm]

It'd be cool if blocks could only be updated every so often, so as to prevent long operations from being run all the time. For example, in our product markdown-inject runs on precommit, but it could be beneficial to have some emitted blocks of some components' npm run build output. It's not really something that changes so often that it need be evaluated every time, but running it every month or two could be nice.

<!--
  CODEBLOCK_START
  {
    "type": "command",
    "debounce": "1 month",
    "value": "npm run build"
  }
-->

To reduce complication / dependencies, the debounce property could instead take a number of minutes (43200), seconds (2592000), milliseconds (2592000), or something.

Then, markdown-inject would automatically inject a last-update property into the JSON config when it runs for that block:

<!--
  CODEBLOCK_START
  {
    "type": "command",
    "debounce": "1 month",
    "value": "npm run build",
    "last-update": "2021-11-23T22:43:35.339Z"
  }
-->

Preferably this date would be captured with new Date().toISOString() so the date is both easily machine parseable, but also decently human readable.