Add trigger character to SignatureHelpProvider

[mjbvz]

Problem
Some signature help providers need to know why they are triggered. See microsoft/TypeScript#24915 for more details on one use case

Proposal
Pass the triggerCharacter to the provideSignatureHelp call. This could be introduced in a context parameter at the end of the parameter list:

export interface SignatureHelpContext {
    triggerCharacter?: string;
}

export interface SignatureHelpProvider {

	provideSignatureHelp(document: TextDocument, position: Position, token: CancellationToken, context: SignatureHelpContext): ProviderResult<SignatureHelp>;
}

[mjbvz]

Moving to more generic proposal: #54972