feat: support custom source map implemetation

[intellild]

Summary

This refactors CSS printing so ToCss depends on a sealed printer trait instead of the concrete Printer<W, S> type. It also splits source map writing behind custom_sourcemap, while keeping the existing sourcemap feature backed by Parcel source maps for current users.

Why

Bundlers such as Rspack already have their own source map representation. Previously, they had to generate a Parcel source map from Lightning CSS output and then translate it back into their own source map type.

With this change, those consumers can provide a custom source map writer directly.

This has two benefits:

• Reduces binary size by avoiding the built-in Parcel source map implementation when it is not needed.
• Improves CSS printing performance by avoid source map translation.

In Rspack CI, this reduced the binary by about 400 KB:
https://github.com/web-infra-dev/rspack/actions/runs/28227945651/job/83627257783?pr=14581

Release builds are estimated to save around 300 KB.

Compatibility

The default sourcemap feature still uses Parcel source maps, so existing Lightning CSS users should keep the same public behavior.

Consumers that want to avoid the built-in Parcel implementation can enable custom_sourcemap and provide their own writer.

Validation

Checked the library with and without custom source maps, including no-default-feature and serde-only configurations.

[devongovett]

Sounds like a good idea. I'll look more in depth soon.

[intellild]

@devongovett Should we maintain rust API compatibility across the 1.0.0 beta releases?