Fix enum strings used in error messages

[jakebailey]

Noticed these while working on another PR. Yay reverse mapping. Good thing our error messages use uppercase form?

[Copilot]

Pull Request Overview

This PR updates error messages to use uppercase enum strings and adjusts the stringer code generation for enum representations. Key changes include:

• Revision of baseline error message diffs for various module targets (nodenext, node18, node16) to reflect the new enum names.
• Removal of the combined compileroptions_stringer_generated.go file and introduction of separate generated files for ModuleKind and ScriptTarget.
• Update of go:generate directives in internal/core/compileroptions.go to align with the new generation strategy.

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/baselines/reference/submodule/conformance/nodeModulesJson(module=nodenext).errors.txt.diff	Updated error messages to reference the new ModuleKindNodeNext enum string.
testdata/baselines/reference/submodule/conformance/nodeModulesJson(module=nodenext).errors.txt	Revised error messages for NodeNext to new expected enum representations.
testdata/baselines/reference/submodule/conformance/nodeModulesJson(module=node18).errors.txt.diff	Adjusted error messages to update references from Node18 to ModuleKindNode18 where applicable.
testdata/baselines/reference/submodule/conformance/nodeModulesJson(module=node18).errors.txt	Revised error baseline for Node18 with corrected enum strings.
testdata/baselines/reference/submodule/conformance/nodeModulesJson(module=node16).errors.txt.diff	Modified error messages to use Node16 instead of ModuleKindNode16.
testdata/baselines/reference/submodule/conformance/nodeModulesJson(module=node16).errors.txt	Revised Node16 error messages to reflect the new enum naming.
internal/core/scripttarget_stringer_generated.go	New generated file for ScriptTarget enum string conversion.
internal/core/modulekind_stringer_generated.go	New generated file for ModuleKind enum string conversion.
internal/core/compileroptions_stringer_generated.go	Removed outdated generated file in favor of separate ones.
internal/core/compileroptions.go	Updated go:generate directives to generate separate stringer files with the proper trimprefix settings.