[Feature Request] Configuring the output namespace for init_expressions

[sylenthuntress]

When I'm developing, I try to keep the namespace as generic as possible at first and then rename it later. This results in a lot of unused constants from foo:init_expressions later down the line. In the case where my datapack will be alone (i.e mapmaking) I would like to move it to a different, consistent namespace like main:init_expressions.

[rx-dev]

You can do this via Expression.init