refactor: simplify IsPredeclaredGoIdentifier implementation

[alexandear]

This PR refactors codegen.IsPredeclaredGoIdentifier implementation to use the function doc.IsPredeclared from std.

[jamietanna]

Hi, appreciate you've worked on this and it's not yet in - please bear with, we're over-stretched and are trying to prioritise higher priority pieces (when we get time to work on the project).

Once we get through the backlog for the next release we'll revisit this

[alexandear]

Closing due to inactivity.