add isBigint and isBigintLiteral()

[mjperrone]

https://ts-morph.com/details/types#telling-type

If you see something that doesn't exist here and should (there's a lot missing), then please log an issue or submit a pull request.

Is your feature request related to a problem? Please describe.

I'd like to write some code that behaves differently when it encounters a bigint typed node.

Describe the solution you'd like

those type guard functions defined

This is a likely implementation, but I'm not sure I'd be able to get the surrounding work done adequately (docs, tests, build etc) so submitting this as an issue to ask for help.

  /**
   * Gets if this is a bigint literal type.
   */
  isBigIntLiteral(): this is Type<ts.BigIntLiteralType> {
    return this.#hasTypeFlag(TypeFlags.BigIntLiteral);
  }

[mjperrone]

@dsherret is this something you would consider doing or giving advice on?