feat: Implement slice_push_back in the comptime interpreter

[jfecher]

Description

Problem*

Summary*

Implements slice_push_back in the comptime interpreter. This was needed in the vertical slice example to build a slice of quoted fields for the type in the impl.

Additional Context

With this a slightly modified version of the vertical slice code now runs. Notably:

• Generic functions do not work, so a map function like in the original cannot be used
• An annotation function can return a quoted impl, but the impl isn't actually inserted into the program yet. So the derived impl does nothing currently.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.