Add nullptr literal with pointer coercion

[grian32]

Goal

Add a real null pointer literal so code can write null pointer values without (0 as T*), and so C/exdef null constants have a clean representation.

Tasks

• Decide token spelling, likely nullptr.
• Add lexer/parser/AST support.
• Checker: allow coercion only into pointer contexts.
• Emitter: lower to null pointer of target type.
• Update exdef handling for ((void*)0) style constants.
• Add syntax/docs examples.

Refs: TODO.md, cli/exdef.go:63.

Related: #4