Baseline status lookup

[smsnedden]

This adds a standalone lookup that maps MDN Browser Compatibility Data (BCD) keys to their Baseline status using the web-features package.

The HTML, CSS, and JavaScript detectors produce BCD keys, so this provides a shared, detector-independent classification step. Connecting the lookup to those detectors and producing status-grouped output will follow in later changes.

Changes

Baseline status lookup

Adds lookupBaselineStatus, which:

• Builds a BCD-key-to-feature-ID index once for constant-time lookups.
• Handles the feature, moved, and split records provided by web-features.
• Prefers per-compat-key status when available, falling back to feature-level status.
• Returns null for unknown BCD keys.
• Preserves the classification supplied by web-features, including false, without reclassification.
• Includes feature names, Baseline dates, browser support, and discouraged-feature metadata.
• Returns independent copies of mutable metadata so callers cannot affect subsequent lookups.

Result types

Adds types describing Baseline classifications, supported browsers, discouraged-feature metadata, and lookup results:

export interface BaselineLookupResult {
  featureId: string;
  featureName: string;
  baseline: BaselineStatus;
  baselineLowDate: string | null;
  baselineHighDate: string | null;
  support: BaselineSupport;
  discouraged?: Discouraged;
}

Baseline date strings are returned as represented by web-features, including uncertainty prefixes such as ≤ where applicable.

Dependency

Adds web-features as a runtime dependency. Because it contains evolving Baseline data, keeping classifications current requires periodically updating the resolved version in package-lock.json and reviewing any resulting test changes. The compatible version range permits these updates but does not update existing lockfiles automatically. This PR does not introduce an automated refresh schedule.

Tests

Adds unit coverage for:

• Widely available and Newly available features.
• Unknown BCD keys.
• Preserving an unclassified false status.
• Per-compat-key status precedence.
• Discouraged-feature metadata.
• Isolation of returned metadata from caller mutation.
• Omission of discouraged metadata for ordinary features.

[nicjansma]

nit: was this "unwound" to a single line from a formatter, or manually?

(I'd generally prefer each on its own line for readability and maintainability, when an item gets added or removed it just affects a single line).

[smsnedden]

It was unwound from a formatter, I'll restore the existing one-per-line formatting now.

[sergeychernyshev]

Let's use prettier linter to format code and make sure we codify our preferences as rules.

[sergeychernyshev]

Adding more types would help with maintenance in the future.

[sergeychernyshev]

Let's define types for these variables so we don't get some weird data as we update the web-features package.