Allow combining `select[Label]`s

### Description of the feature request:

With the new symbolic macros, it's [difficult to manipulate](https://bazel.build/docs/configurable-attributes#faq-select-macro) `attr.label` inputs, as they're wrapped in `select[Label]` objects. Allowing the following operation would regain a lot of functionality: `select[Label] + select[Label] -> select[list[Label]]`.

### Which category does this issue belong to?

Configurability, Starlark Interpreter

### What underlying problem are you trying to solve with this feature?

To retain flexibility in making labels a list of labels within a macro.

### Which operating system are you running Bazel on?

Ubuntu 24.04

### What is the output of `bazel info release`?

release 8.0.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text
git@github.com:RyanDraves/nlb.git
b33e99e0d05e3016ca92680b186479c7b0ca0e2e

I removed the symbolic macros; I can make a branch re-adding them if requested. A full example is on [Slack](https://bazelbuild.slack.com/archives/CA31HN1T3/p1733981276254349?thread_ts=1733981177.926289&cid=CA31HN1T3).
```

### Have you found anything relevant by searching the web?

Relevant docs: https://bazel.build/docs/configurable-attributes#faq-select-macro
Relevant proposal: https://docs.google.com/document/d/1c-8GNkz57pDgBGPdJoXJ42aCFHr-gN-Ebon9Zx1COsk/edit?tab=t.0#heading=h.5mcn15i0e1ch

### Any other information, logs, or outputs that you want to share?

_No response_

Start with the configurable attributes FAQ and the linked proposal, then reproduce the reported limitation using the symbolic-macro example referenced in the issue. Determine the intended behavior for combining select[Label] values into select[list[Label]], and confirm that the implemented behavior preserves configurability and label typing.

Written by the indexing model from the issue text.