Remove ES5 references, misc cleanup

[jakebailey]

Some cleanup after #63067.

This removes all remaining non-test references to ScriptTarget.ES5. Most are just dead code or can be rephrased.

The one behavior difference is in the third commit. For enums, we were being conservative and printing identifiers that are only valid in ES2015+ as string literals. However, this does not practically matter because declaration emit just reuses the nodes, so everyone has been emitting these characters anyway. That and, we always parse declaration files with the latest options such that they're always cached (fairly certain; definitely true in Corsa), so it doesn't matter there. All that changes is the types baseline, which also implies that hover would change, which it does.

Strada before:

Corsa today (what this PR would do):

And of course, Corsa does not even have the code to do it the old way, lacking ES5 stuff or the plumbing to even pass that down.

[Copilot]

Pull request overview

This pull request removes remaining non-test references to ScriptTarget.ES5 after the deprecation of ES5 as a compilation target. The PR includes cleanup changes across the codebase and adds a new test to document the behavior change for enum member name validation.

Changes:

• Changed identifier text validation from ES5 to ESNext standard in checker.ts, affecting how enum member names are validated for type representation
• Simplified conditions in transformers and emitter that previously checked for ES5, now using ES2015 or removing the check entirely
• Added comprehensive test case demonstrating enum member name behavior with special characters

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
tests/cases/compiler/enumMemberNameNonIdentifier.ts	New test file demonstrating enum member name handling for non-standard identifiers
tests/baselines/reference/enumMemberNameNonIdentifier.types	Baseline showing type representation for enum members with special character names
tests/baselines/reference/enumMemberNameNonIdentifier.symbols	Baseline showing symbol information for enum members with special character names
tests/baselines/reference/enumMemberNameNonIdentifier.js	Baseline showing JavaScript and declaration file output for enum test
src/compiler/transformers/ts.ts	Changed class IIFE promotion check from <= ES5 to < ES2015
src/compiler/emitter.ts	Removed ES5 version check for trailing comma support in object literals
src/compiler/checker.ts	Changed enum member identifier validation from ES5 to ESNext; removed ES5 check in class name collision detection