feat: Replace + icon with edit icon for New Task button

[ghost]

Description

This PR addresses Issue #7941 by replacing the "+" icon with a compose/pencil icon for the "New Task" button in the top bar.

Changes

• Changed icon from $(add) to $(edit) in src/package.json for the plusButtonClicked command
• Updated test expectations in packages/build/src/__tests__/index.test.ts to reflect the icon change

Motivation

The edit/pencil icon better conveys "start composing a task" rather than "add", providing a more intuitive user experience.

Testing

• ✅ All tests pass
• ✅ Linting checks pass
• ✅ Type checking passes

Screenshots

The $(edit) icon is a standard VS Code codicon that displays as a pencil/compose icon and is designed to work well across all themes (light, dark, and high-contrast).

Fixes #7941

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Important

Replaces the '+' icon with a pencil icon for the 'New Task' button, updating src/package.json and test expectations in index.test.ts.

• Behavior:
  • Replaces $(add) icon with $(edit) icon for plusButtonClicked command in src/package.json.
  • Updates test expectations in index.test.ts to reflect icon change.
• Motivation:
  • The pencil icon better conveys the action of composing a new task, improving user experience.
• Testing:
  • All tests, linting, and type checks pass.

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