Add label management

[swinton]

Labels are Gmail's primary organizational primitive and are completely absent from the REPL today.

Proposed API

// List all labels
await labels()

// Get a single label
await label('Label_123')

// Add a label to a message
await addLabel('1883a5111e8ef47c', 'Label_123')

// Remove a label from a message
await removeLabel('1883a5111e8ef47c', 'Label_123')

Notes

• users.labels.list
• users.labels.get
• users.messages.modify — used for both add and remove via addLabelIds / removeLabelIds
• Worth exposing a labelId(name) helper to resolve human-readable label names to IDs, since the API uses opaque IDs