Anthropic Claude 3 support (Sonnet, Opus, Haiku, and Vision)

[gogwilt]

• Adds Claude 3 models (Sonnet, Opus, Haiku)
• Uses Anthropic's Messages API for Claude 3 models.
• Adds Claude Vision support via images in messages.
• Updates anthropic.md documentation.

Also: makes it a BIT easier to develop on the node-executor

This was originally PR #369, but I don't have permissions on @wayne-chang's fork.

[gogwilt]

Some potential TODOs:

• [x] System prompts - I need to test locally whether system prompts work as expected.
• [x] Token counts - Anthropic's streaming API actually returns accurate token counts, so we can give more accurate cost estimates.

[gogwilt]

@abrenneke this PR is now ready to go, pending approval.