V2: For deprecated elements, generate a @deprecated tag on descriptors

[timostamm]

Following up on #814 (comment), this adds a @deprecated tag to generated descriptors. For example:

syntax="proto3";

message Example {
  option deprecated = true;
}

Already generates:

/**
 * @generated from message Example
 * @deprecated
 */
export type DeprecatedMessage = Message<"Example"> & {};

With this change, the descriptor is also marked deprecated:

/**
 * Describes the message Example.
 * Use `create(ExampleDesc)` to create a new message.
 * @deprecated
 */
export const ExampleDesc: GenDescMessage<Example> = /*@__PURE__*/
  messageDesc(fileDesc_example, 0);

Previously, we generated line comments for descriptors, but since the @deprecated tag is ignored in line comments, we're switching to block comments.

This also improves implicit deprecation: If a message, extension, or enum is nested in a deprecated message, it is also generated with a @deprecated tag. Similar for files that are marked deprecated.

[timostamm]

Generates the block comments for descriptors.

[timostamm]

Improves existing behavior for implicit deprecation.