Document environment-conditioned assets

[Calpoog]

Description of what this PR is changing or adding, and why:

Documents the proposed environment subfield for Flutter asset declarations. The assets guide explains exact matching, AND semantics across variables, OR semantics for value lists, unset-variable behavior, and composition with flavor and platform conditions. The pubspec reference defines the field and includes scalar and list examples.

Issues fixed by this PR (if any):

Supports flutter/flutter#166842.

PRs or commits this PR depends on (if any):

Blocked by flutter/flutter#192739. This documentation should remain draft until the framework feature is accepted and its release availability is known.

Validation

• dart run dash_site --site=docs build
• dart run dash_site --site=docs check-link-references
• dart run dash_site --site=docs check-links

check-all also completed tests and excerpt validation, but its analyzer process crashed while attempting to send analytics because the network connection was cancelled. Its format check reports three pre-existing unrelated Dart files.

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks of 80 characters or fewer.

[google-cla]

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