Add intEnum support, tests for enum generation

[srchase]

PR adds support for intEnum shapes added with Smithy IDLv2. It relies DirectedCodegen changes added in Smithy 1.26.0 and updates to use 1.26.x.

Given this Smithy model:

structure MyStruct {
    myIntEnum: MyIntEnum
}

intEnum MyIntEnum {
    BAR = 3
    FOO = 1
    BAZ = 2
}

The following types will be generated:

export enum MyIntEnum {
  FOO = 1,
  BAZ = 2,
  BAR = 3,
}

export interface MyStruct {
  myIntEnum?: MyIntEnum | number;
}

This PR also adds and updates tests and documentation for enum shape generation, including the | string behavior in the SymbolVisitor, and ordering applied to members in EnumGenerator.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jhecking]

This was merged in October, but there hasn't been a release of the smithy-aws-typescript-codegen package since 0.12.0 in September. I would just like to understand what the intention & recommendation of the AWS Labs team is with regards to this package: At what point will a new official package be released? And should we rather build this package directly from the main branch ourselves rather than rely on the official package releases if we want to use it to generate our own Typescript client/server SDKs using Smithy?