TypeSpec initializer comment support

[Scottmitch]

Motivation:
When generating code it maybe useful to insert
comments as anchors for post processing and further extension by other code generation tools. However this is not easy to achieve with the existing API.

Modifications:

Add a TypeSpec.addInitializerComment method which is emitted only as comments above the addInitializerBlock.

[palantirtech]

Thanks for your interest in palantir/javapoet, @Scottmitch! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[changelog-app]

Generate changelog in changelog/@unreleased

What do the change types mean?

• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

Note: only one type should be chosen.

How are new versions calculated?

• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

TypeSpec initializer comment support

Check the box to generate changelog(s)

• Generate changelog entry

[Scottmitch]

Moved from square/javapoet#967.

Still interested in this feature, can someone advise on merging?