Add local development instructions to README

[shinshin86]

This PR adds a new "Development" section to the README, providing step-by-step instructions for building and testing sqlc-gen-typescript locally. These additions aim to:

1. Guide developers through the process of local setup and testing
2. Lower the barrier to entry for potential contributors

By making the development process more transparent and accessible, we hope to encourage more developers to contribute to this project.

[shinshin86]

I overlooked that the build process was already defined in the Makefile.
Therefore, I've updated the WASM build instructions to reference the Makefile.
Additionally, I've corrected the wasm file specification which previously assumed an absolute path with file:///.
These changes should make the development process clearer and more flexible for contributors.

[yshrsmz]

Maybe we should add a link to the core sqlc development guide
https://docs.sqlc.dev/en/latest/guides/development.html

[shinshin86]

@yshrsmz
Thank you for your valuable feedback! I've added a link to the core sqlc development guide as per your suggestion.
If you have any further suggestions or feedback, please let me know. Thanks again!