Global process=false flag

[jmduke]

We're porting our docs to Markdoc (backed by Keystatic) and got bitten pretty hard by the issue described in #72 and #306 because our docs have a lot of handlebar-ish code blocks, and it's somewhat hard to enforce the process=false tag in every single code block. @rpaul-stripe mentioned adding a global configuration for this and I'm opening this issue to formally suggest that.

(I would also argue that the default should probably be process=false rather than process=true, but that's a different discussion!)

[mfix-stripe]

@jmduke you can handle this by creating a custom fence Node and setting the new default.

Pseudocode:

import {nodes} from '@markdoc/markdoc'

// ...
nodes: {
  fence: {
    ...nodes.fence,
    attributes: {
      ...nodes.fence.attributes,
      process: { ...nodes.fence.attributes.process, default: false },
    }
  }
}