Add src-output-dir option for specifying source code output dir

[dgetu]

Example usage:

Existing customization behavior should stay the same - the default behavior is to emit to generated if the dir already exists, src otherwise.

{
  "src-output-dir": "src/generated", // defaults to "generated" if the directory exists at emit time, "src" otherwise
  "entry-point-root": "src" // defaults to "src-output-dir" if defined, "src" otherwise
}

ls .

filename	example filename	kind
${entry-point-root}/index.ts	src/index.ts	authored
${entry-dir}/models/index.ts	src/models/index.ts	authored
${entry-dir}/api/index.ts	src/api/index.ts	authored
${src-output-dir}/index.ts	src/generated/index.ts	generated
${src-output-dir}/models/...	src/generated/models/...	generated
${src-output-dir}/api/...	src/generated/api/...	generated

If entry-point-root isn't the same as src-output-dir, create each missing authored file of the three listed above, and re-export the corresponding generated index.ts from it. If an authored file already exists, don't change it, since it's meant to be customized by the SDK author.

src/index.ts (only write to this file if it doesn't exist)

export * from "./generated/index.js";

package.json, api-extractor.json, tsconfig.json and .tshy config output need to be updated as well.

[qiaozha]

Hi @dgetu if I remember correctly, the customization process v2 has changed the folder structure like

- generated
- src 

Customization code will go into src folder directly, and we will run npx dev-tool customization apply-v2 to combine/merge the generated code into the src folder. So that there's no direct reference to the generated folder. You may refer to this loop for more context and I think @timovv is the one who writes the loop.