Add --marker flag to uv add (#12012)

## Summary

Add a `--marker` flag to `uv add` which applies a marker to all given
requirements.

Example:

```
$ uv-debug add --marker "platform_machine == 'x86_64'" \
    "anyio>=2.31.0" \
    "iniconfig>=2; sys_platform != 'win32'" \
    "numpy>1.19; sys_platform == 'win32'"
```

```toml
[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.12.0"
dependencies = [
    "anyio>=2.31.0 ; platform_machine == 'x86_64'",
    "iniconfig>=2 ; platform_machine == 'x86_64' and sys_platform != 'win32'",
    "numpy>1.19 ; platform_machine == 'x86_64' and sys_platform == 'win32'",
]
```

Fixes https://github.com/astral-sh/uv/issues/11987


## Test Plan

Added snapshot tests

---------

Co-authored-by: konstin <konstin@mailbox.org>

docs/reference/cli.md

@@ -951,6 +951,8 @@ uv add [OPTIONS] <PACKAGES|--requirements <REQUIREMENTS>>
 <p>Requires that the lockfile is up-to-date. If the lockfile is missing or needs to be updated, uv will exit with an error.</p>
 
 <p>May also be set with the <code>UV_LOCKED</code> environment variable.</p>
+</dd><dt id="uv-add--marker"><a href="#uv-add--marker"><code>--marker</code></a>, <code>-m</code> <i>marker</i></dt><dd><p>Apply this marker to all added packages</p>
+
 </dd><dt id="uv-add--native-tls"><a href="#uv-add--native-tls"><code>--native-tls</code></a></dt><dd><p>Whether to load TLS certificates from the platform&#8217;s native certificate store.</p>
 
 <p>By default, uv loads certificates from the bundled <code>webpki-roots</code> crate. The <code>webpki-roots</code> are a reliable set of trust roots from Mozilla, and including them in uv improves portability and performance (especially on macOS).</p>