Change default target from ES5 to ESNext

[Copilot]

This PR changes the default TypeScript compilation target from es5 to esnext to better align with modern JavaScript environments and "evergreen runtimes" that are now the norm.

Changes Made

• Updated the default target fallback in src/compiler/utilities.ts from ScriptTarget.ES5 to ScriptTarget.ESNext
• Updated the help documentation default in src/compiler/commandLineParser.ts from ScriptTarget.ES5 to ScriptTarget.ESNext
• Accepted updated test baselines reflecting the new ESNext output instead of ES5 downleveling

Impact

When no explicit --target is specified, TypeScript will now:

• Preserve modern JavaScript syntax (classes, arrow functions, spread operators, etc.)
• Generate cleaner, more readable output
• Target modern JavaScript environments by default

Before (ES5 default):

// Input
const x = [1, 2, 3];
console.log(...x);

// Output (downleveled)
var x = [1, 2, 3];
console.log.apply(console, x);

After (ESNext default):

// Input
const x = [1, 2, 3];
console.log(...x);

// Output (preserved)
const x = [1, 2, 3];
console.log(...x);

Migration

This is a breaking change. Users who need ES5 output for compatibility with older environments should explicitly specify --target es5 in their tsconfig.json:

{
  "compilerOptions": {
    "target": "es5"
  }
}

Tests with explicit // @target: es5 directives continue to work correctly and are unaffected.

Rationale

• ES5 will be deprecated in TypeScript 6.0 and removed in 7.0 (Deprecate --target es5, make lowest target es2015 #62196)
• Modern JavaScript environments support ESNext features natively
• Reduces unnecessary code transformation and improves output quality
• Aligns with TypeScript's core value proposition as "JavaScript with type syntax"

Fixes #62198.

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