Add a new baseline lib

[3ru]

Fixes #62536

Overview

• This PoC adds a new lib value, baseline, to TypeScript.
• When users set compilerOptions.lib: ["baseline"] in tsconfig.json, the compiler only exposes types for JavaScript built‑ins that have reached Baseline Widely Available ("high"). Any built‑ins that are not yet in Baseline (low/false) are left undeclared and thus surface as ordinary type errors when referenced.
• Language syntax (operators/statements/grammar/classes) is out of scope for this PR.

Scope (In/Out)

• In: ECMAScript (JS) built‑in APIs only.
• Data source: Features under javascript.builtins.* in web‑features whose status.baseline == "high" (Widely Available).
• Out: Language syntax (operators/statements/grammar/classes), DOM/Web API/Worker/ScriptHost (host environment APIs). (TODO)

Spec

• baseline.
  • Concretely, we introduce src/lib/baseline.d.ts, and reference only those existing src/lib/es20xx.*.d.ts fragments that correspond to Baseline Widely features via /// <reference lib="...">.
  • Built‑ins that haven't reached Baseline remain undeclared, so referencing them produces normal type errors.

Data Source and Verification (jq)

• Data: web-features data.json
  • Path: /Users/ru/Documents/eslint-plugin/eslint-plugin-baseline-js/node_modules/web-features/data.json
• We extract feature IDs where status.baseline == "high" and any compat_features entry starts with javascript.builtins.. We verified both the count and the list.

Set a variable:

DATA_PATH="/Users/ru/Documents/eslint-plugin/eslint-plugin-baseline-js/node_modules/web-features/data.json"

Count (unique feature IDs):

jq -r '.features | to_entries[] \
  | select(.value.status?.baseline=="high") \
  | select(any(.value.compat_features[]?; startswith("javascript.builtins."))) \
  | .key' "$DATA_PATH" | sort -u | wc -l

List (unique feature IDs):

jq -r '.features | to_entries[] \
  | select(.value.status?.baseline=="high") \
  | select(any(.value.compat_features[]?; startswith("javascript.builtins."))) \
  | .key' "$DATA_PATH" | sort -u

• At the time of writing, we extract 65 JS built‑in features as Baseline Widely.

TODO

• Scope and variants

  • Should we offer a baseline.full variant similar to es2015.full that also includes host libs like dom? (Note: this would mean JS is Baseline‑gated, but DOM is not.)

• Defining baseline.d.ts

  • Should we keep the allowlist references in the TS repo manually, or provide a mapper/tooling on the WebDX CG side? Either way, this repo needs the final mapping.
  • Known mismatch example between web‑features granularity and TS lib fragment granularity:
    • src/lib/es2023.array.d.ts currently contains both:
      • findLast/findLastIndex (web‑features: array-findlast, Baseline = high), and
      • toReversed/toSorted/toSpliced/with (web‑features: array-by-copy, Baseline = low)
    • We should decide whether to split this into separate fragments (e.g. es2023.array.findlast vs es2023.array.by-copy) so baseline can safely include only the Baseline‑high set.

• Sustainable data maintenance