Add unfurl_links and unfurl_media to Message

[tylerhammer]

Is your feature request related to a problem? Please describe.
Messages with links are unfurling in channels where they should not be. Have been passing in these parameters but they don't make it to the final message

Describe the solution you'd like
Please add unfurl_links and unfurl_media to Message + MessageParams

slack-block-builder/src/surfaces/message.ts

Lines 29 to 34 in bdb89d7

	export interface MessageParams {
	channel?: string;
	postAt?: number;
	text?: string;
	threadTs?: string;
	ts?: string;

Describe alternatives you've considered
Unable to get around it as object is non-extensible

Additional context
Slack API https://api.slack.com/methods/chat.postMessage