Disallow [await] using statements under case/default clauses

[Copilot]

Port of microsoft/TypeScript#62709.

Per tc39/ecma262@8e5d501, using and await using variable statements cannot be immediate children of case/default clauses—they must be wrapped in a block.

switch (x) {
    case 0:
        using d = expr;     // TS1547: not allowed
        break;
    case 1: {
        using d = expr;     // ok, wrapped in block
        break;
    }
}

• Checker: Added grammar check in checkGrammarVariableDeclarationList emitting TS1547/TS1548 when using/await using declarations appear directly under case/default clauses
• Transformer: Removed visitSwitchStatement, visitCaseOrDefaultClause, and getUsingKindOfCaseOrDefaultClauses from the esnext using-declaration transformer, since these are now grammar errors and never reach emit

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[Copilot]

Pull request overview

Ports TypeScript behavior to treat using / await using variable statements as grammar errors when they appear directly under case/default clauses (must be wrapped in a block), aligning the Go port’s checker and emit pipeline with upstream TS.

Changes:

• Added a grammar check in checkGrammarVariableDeclarationList to emit TS1547/TS1548 for using/await using directly under case/default.
• Removed switch/clause-specific handling from the ESNext using-declaration transformer since such constructs are now grammar errors.
• Updated reference baselines for affected submodule conformance tests.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/grammarchecks.go	Emits TS1547/TS1548 when a using/await using variable statement is directly under a case/default clause.
internal/transformers/estransforms/using.go	Removes switch/case-clause transformation logic that is no longer reachable for valid programs.
testdata/baselines/reference/submodule/conformance/usingDeclarations.17.errors.txt.diff	Removes obsolete diff baseline output.
testdata/baselines/reference/submodule/conformance/usingDeclarations.17.errors.txt	Adds/updates expected TS1547 error baseline.
testdata/baselines/reference/submodule/conformance/awaitUsingDeclarations.17.errors.txt.diff	Removes obsolete diff baseline output.
testdata/baselines/reference/submodule/conformance/awaitUsingDeclarations.17.errors.txt	Adds/updates expected TS1548 error baseline.