Add support for exporting default keybindings

[ntrogh]

Introduce functionality to export default keybindings for all platforms via a CLI option:

code --export-default-keybindings <output folder>

Ref: #299021

[Copilot]

Pull request overview

This PR introduces a command-line option (--export-default-keybindings) to export default keybindings for all three platforms (Windows, macOS, Linux) as JSON files. This is needed to generate documentation reference files that map command IDs to their default keyboard shortcuts.

Changes:

• Adds a new KeybindingsExportContribution that reads the CLI flag and writes three per-platform keybinding JSON files, then quits
• Refactors KeybindingsRegistryImpl to also store raw IKeybindingRule objects in a new _coreKeybindingRules list, enabling re-resolution of keybindings for any target OS
• Adds getDefaultKeybindingsContentForOS(os) to IKeybindingService and implements it in WorkbenchKeybindingService using per-OS keyboard mappers built from the exported EN-US layout constants

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/vs/workbench/workbench.desktop.main.ts	Registers the new keybindingsExport contribution for desktop builds
src/vs/workbench/contrib/keybindingsExport/electron-browser/keybindingsExport.contribution.ts	New contribution: reads --export-default-keybindings CLI flag, writes platform-specific JSON files, then calls nativeHostService.exit
src/vs/workbench/services/keybinding/browser/keybindingService.ts	Adds getDefaultKeybindingsContentForOS, _createKeyboardMapperForOS, and _resolveKeybindingItemsWithMapper static helpers
src/vs/workbench/services/keybinding/browser/keyboardLayouts/en.win.ts	Exports EN_US_WIN_LAYOUT constant so it can be imported by the keybinding service
src/vs/workbench/services/keybinding/browser/keyboardLayouts/en.linux.ts	Exports EN_US_LINUX_LAYOUT constant
src/vs/workbench/services/keybinding/browser/keyboardLayouts/en.darwin.ts	Exports EN_US_DARWIN_LAYOUT constant
src/vs/platform/keybinding/common/keybindingsRegistry.ts	Adds _coreKeybindingRules list to store raw rules; adds getDefaultKeybindingsForOS; refactors bindToCurrentPlatform to delegate to new bindToPlatform(kb, os)
src/vs/platform/keybinding/common/keybinding.ts	Adds getDefaultKeybindingsContentForOS(os) to IKeybindingService interface
src/vs/platform/keybinding/common/abstractKeybindingService.ts	Provides default no-op getDefaultKeybindingsContentForOS implementation
src/vs/platform/keybinding/test/common/mockKeybindingService.ts	Adds stub getDefaultKeybindingsContentForOS to the mock
src/vs/platform/environment/common/environment.ts	Adds exportDefaultKeybindings?: string to INativeEnvironmentService
src/vs/platform/environment/common/environmentService.ts	Implements exportDefaultKeybindings getter
src/vs/platform/environment/common/argv.ts	Adds 'export-default-keybindings'?: string to NativeParsedArgs
src/vs/platform/environment/node/argv.ts	Registers the --export-default-keybindings option descriptor with allowEmptyValue: true

[ntrogh]

@joaomoreno Could you review this PR to verify if there are no obvious engineering inconsistencies or possible improvements? Thanks!