feat: add missing-typography lint rule

[chelseayerong]

Description

This PR adds a new lint rule missing-typography to the design.md linter.

Rationale

The current missing-sections rule checks for missing spacing and rounded sections when colors are present, but it does not check for typography. Typography is a core part of a design system; without explicit typography tokens, consumers are forced to fall back to default fonts, reducing the author's control over the design system's identity.

Changes

• Added missing-typography rule in packages/cli/src/linter/linter/rules/missing-typography.ts.
• Added unit tests in packages/cli/src/linter/linter/rules/missing-typography.test.ts.
• Registered the new rule in the default rules list.
• Updated README.md to document the new rule.
• Updated existing tests in types.test.ts and spec.test.ts to account for the new rule count (8 instead of 7).

Verification

Ran bun test and all 185 tests passed successfully.

[davideast]

This is awesome!