native: split codegen into multiple files and refactor assign statement generation

[Spydr06]

In continuation of #17782:

• Splitting up the code into multiple files (gen.v, expr.v, stmt.v) makes navigating much easier.
• Assignment statements now don't rely on hard-coded initializer expressions, but can take values from every expression classified trough the type-system

🤖 Generated by Copilot at 951f5e3

This pull request implements the basic infrastructure and logic for generating native code for expressions and statements in the V language, and adds a test program and its expected output for the native backend. It also adds a license header and a stub function for the ARM64 code generator.

🤖 Generated by Copilot at 951f5e3

• Add license header to vlib/v/gen/native/arm64.v (link)
• Add stub function for init_array in Arm64 struct (link)
• Add vlib/v/gen/native/expr.v with logic for expression code generation (link)