Add runtime resource parser + Improve CST resource parser - #471

/**
 * Parse input into a tree of MessageFormat instances.
 *
 * The frontmatter locale is required;
 * all other metadata is ignored.
 *
 * @param source - The full source being parsed
 * @param options - The options used for each MessageFormat isntance.
 */
export function parseMessageResource(
  source: string,
  options?: MessageFormatOptions
): MessageResource;

/**
 * Compile a `CST.Resource` value into a tree of MessageFormat instances.
 *
 * @param cst - The previously parsed CST resource
 * @param options - The options used for each MessageFormat isntance.
 */
export function buildMessageResourceFromCST(
  cst: CST.Resource,
  options?: MessageFormatOptions
): MessageResource;