Write a migration guide covering all breaking changes in 6.0.0

[stefanobaghino]

Write a migration document covering all breaking changes in the 6.0.0 release. The document should:

• List every removed, renamed, or changed public API item with before/after code snippets.
• Cover the nonewlines removal (Abandon nonewlines for v6.0.0 #394) with the recommended replacement pattern.
• Explain the new error types that replace leaked dependency types (Remove dependencies from public API #512).
• Document the SyntaxReference.variables change (refactor: Move variables into compressed contexts #540/Consider compressing SyntaxReference's variables #482) and how to access variable data in the new API.
• Document the dep: feature flag changes (Use dep: prefix for feature dependencies #588) and which Cargo.toml feature entries need updating.
• Document the HighlightLines::new() signature change (Don't Panic! #564) and how to construct HighlightOptions.
• Document the new MatchOperation::Embed variant and Pattern::IncludeWithPrototype.
• If Support advanced scope selectors #36 is included, document the new ScopeSelector/ScopeSelectors struct layout.
• If Public dependency on once_cell #575 is included in 6.0.0, document the SCOPE_REPO removal or type change.

This guide should be published alongside the 6.0.0 release (e.g., in CHANGELOG.md or a dedicated MIGRATION.md). A major version with multiple breaking changes across error types, public structs, function signatures, and feature flags needs clear upgrade documentation to avoid trial-and-error migrations. The guide should be written as changes land (not retroactively) so nothing is missed. Suggested for the syntect 6.0.0 milestone.