ref: Rename preamble as startBlock

[RobertGemmaJr]

• Renames preamble as startBlock to match the naming convention of other nested timelines
• Moves camera setup trials into startBlock
• Changes the order of the trials in startblock:
  1. The name of the experiment is displayed
  2. The experiment enters fullscreen
  3. A welcome message is displayed
  4. Trials used to set up a photodiode and trigger box are displayed (if applicable)
  5. Trials used to set up the user's camera are displayed (if applicable).

closes #366

[github-actions]

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