[folding] Multiple FoldingProviders are called...

[jrieken]

re #48352

The doc says that only one folder provider is called, in fact multiple are called and their results are merged.

 * Multiple folding can be registered for a language. In that case providers are sorted
 * by their [score](#languages.match) and the best-matching provider is used. Failure
 * of the selected provider will cause a failure of the whole operation.
 *

    vscode.languages.registerFoldingRangeProvider({ pattern: '**/*.foo' }, new class implements vscode.FoldingRangeProvider {
        provideFoldingRanges(document: vscode.TextDocument, context: vscode.FoldingContext, token: vscode.CancellationToken): vscode.ProviderResult<vscode.FoldingRange[]> {
            return [
                new vscode.FoldingRange(0, document.lineCount - 1, vscode.FoldingRangeKind.Region)
            ]
        }
    });

    vscode.languages.registerFoldingRangeProvider({ pattern: '**/*.foo' }, new class implements vscode.FoldingRangeProvider {
        provideFoldingRanges(document: vscode.TextDocument, context: vscode.FoldingContext, token: vscode.CancellationToken): vscode.ProviderResult<vscode.FoldingRange[]> {
            return [
                new vscode.FoldingRange(1, document.lineCount - 1, vscode.FoldingRangeKind.Region)
            ]
        }
    });

[jrieken]

To make it worst, it will fail the whole operation if one fails...

[aeschli]

made improvements to the spec. in 1.23 (and master)
Keeping open to fix that a failing provider is gracefully handled.