Initialize environment service with isBuilt property in standalone theme service

[Copilot]

The _environment object in StandaloneThemeService was created with Object.create(null), lacking the isBuilt property required by getIconsStyleSheet to determine the correct codicons module path.

Change:

• Initialize _environment with { isBuilt: false } as IEnvironmentService instead of Object.create(null)

Context:
getIconsStyleSheet uses environmentService.isBuilt to select between ASAR unpacked and regular node_modules paths:

export function getModuleLocation(environmentService: IEnvironmentService): AppResourcePath {
	return `${(canASAR && environmentService.isBuilt) ? nodeModulesAsarUnpackedPath : nodeModulesPath}/@vscode/codicons/dist/codicon.ttf`;
}

For standalone editor scenarios (development/browser environments), isBuilt: false ensures the correct non-ASAR path is used.

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