More strict macro API for init vs non init macros

[kLabz]

As a first step towards #7871, adds:

• Context.onAfterInitMacros to register typer-related code from init macros to be run once typer is ready
• Context.initMacrosDone to check if configuration phase is over
• Warnings, hidden behind -D haxe-next for now, about code currently run from init macros that should be delayed using Context.onAfterInitMacros
• Warnings, that were (mostly?) already present with different message/position, about code meant for configuration phase that is currently run after init macros

Also adds Context.getMacroStack() to get a call stack of haxe.macro.Position for code running in macro context. Useful for custom message reporting.

Closes #11039

[kLabz]

(waiting for #11046 before running CI again)

[kLabz]

CI failures are unrelated.