Add GeneratedFile.jsDoc() and other features to @bufbuild/protoplugin

[timostamm]

Since we added exportDecl to to GeneratedFile in #648, it makes sense to add some more convenience methods to make the features more discoverable and require less imports:

  /**
   * Create a string literal.
   */
  string(string: string): Printable;

  /**
   * Create a JSDoc comment block with the given text. Line breaks and white-space
   * stay intact.
   */
  jsDoc(text: string, indentation?: string): JSDocBlock;

  /**
   * Create a JSDoc comment block for the given message, enumeration, or other
   * descriptor. The comment block will contain the original comments from the
   * protobuf source, and annotations such as `@generated from message MyMessage`.
   */
  jsDoc(
    desc: Exclude<AnyDesc, DescFile | DescExtension>,
    indentation?: string,
  ): JSDocBlock;

• GeneratedFile.string replaces the now deprecated literalString from @bufbuild/protoplugin/ecmascript.
• GeneratedFile.jsDoc replaces the now deprecated makeJsDoc and createJsDocBlock from @bufbuild/protoplugin/ecmascript.
• GeneratedFile.export is now deprecated, as it turned out that it has little use in practice. Instead, we export createImportSymbol from @bufbuild/protoplugin/ecmascript now, which is much more flexible.
• Schema has always been available from @bufbuild/protoplugin and @bufbuild/protoplugin/ecmascript, even though it is very much specific to ECMAScript. The former export is now deprecated.

As a diff, these are the changes you should make as a plugin author, to avoid the deprecated types:

- import { Schema } from "@bufbuild/protoplugin";
+ import { Schema } from "@bufbuild/protoplugin/ecmascript";

- import { makeJsDoc, createJsDocBlock } from "@bufbuild/protoplugin/ecmascript";
- import { literalString } from "@bufbuild/protoplugin/ecmascript";

- f.print('const typeName = ', literalString(message.typeName), ';');
+ f.print('const typeName = ', f.string(message.typeName), ';');

- f.print(makeJsDoc(enumeration));
+ f.print(f.jsDoc(enumeration));