CAN should fire a SCRATCHFILE_UPDATED event when the scratchfile is updated

When a scratchfile is created CAN should start to watch it. When CAN detects a change in the scratchfile, a SCRATCHFILE_UPDATED event should be emitted.

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• CAN should parse the scratchfile when it is updated
  • In order to be able to store notes and use content-addressable storage, CAN needs to have access to the note's content. Because of this, CAN should parse the scratchfile whenever it is updated (CAN should fire a SCRATCHFILE_UPDATED event when the scratchfile is updated). Once the AST has been constructed, CAN should ignore 'beneath the fold'.