Prevent parent scope symbols leaking into module's scope

Leaking user-defined symbols from a parent scope to a module's scope can lead to subtle bugs. The module's scope should be as isolated from the parent scope as possible, but not completely (e.g., we still need basic Nushell commands like ls to be visible).

Could be partially addressed by introducing two new FrameTypes:

• Barrier: Stops symbol lookup when traversing the scope stack. This prevents the symbol lookup from searching the parent scopes.
• Prelude: With only Barrier, we'd lose “default” commands that we want to be visible (like ls). Perhaps we could have a Prelude frame type which would get initialized first during create_default_context() with these default commands.

The question is what to do with constants like NU_LIB_DIRS, e.g.,:

const NU_LIB_DIRS = $NU_LIB_DIRS | append '/home/spam/foo`

module {
    use bar.nu  # should it look for 'bar.nu' in '/home/spam/foo'?
}

We might want to decide on this based on the envisioned functionality of how nupm is supposed to use overlays as “virtual environments”, relying on modifying the NU_LIB_DIRS constant.

Start by tracing scope-stack symbol lookup and create_default_context(), then inspect how FrameType and constants such as NU_LIB_DIRS are represented. Decide how Barrier and Prelude frames should interact with module isolation, default commands, and nupm overlays. Done means the lookup rules and NU_LIB_DIRS behavior are specified and implemented with coverage for parent symbols, prelude commands, and module imports.