feat: add tsImportExtension config for TypeScript import paths

[nicolaturcato]

Summary

Adds a new top-level tsImportExtension config option that appends a file extension to relative import paths in generated TypeScript files.

Problem: TypeScript 6 deprecated moduleResolution: "node10", and TypeScript 7 will remove it entirely. Projects using "nodenext" or "node16" resolution require explicit .js extensions on relative imports — but gotsrpc currently generates extensionless imports like:

import * as foo from './vo-constants';

Solution: When tsImportExtension: ".js" is set in the YAML config, generated imports become:

import * as foo from './vo-constants.js';

The option defaults to "" (empty string), so existing behavior is unchanged — this is fully backwards compatible.

Usage

module:
  name: github.com/example/project
  path: ./

tsImportExtension: ".js"

targets:
  # ...
mappings:
  # ...

Changes

• config/config.go — Add TSImportExtension field to Config struct
• internal/build/build.go — Pass conf.TSImportExtension through relativeFilePath() → appended after stripping .ts
• gotsrpc.schema.json — Add tsImportExtension to the JSON schema

Why this matters

• TypeScript 7 will remove moduleResolution: "node10" — projects need "nodenext" or "bundler"
• "nodenext" requires .js extensions on relative imports
• Without this option, consumers must post-process generated files with sed/scripts
• .js extensions are valid across all TS resolution modes, so opting in is always safe