CLI to flatten and generate GQL schema from Solidity file (#245)

* Add schema generation.

* Use yargs and add a script to generate schema.

* Add command to generate schema to README.md

* Remove use of arrow functions in visitor.

* Add function to export schema in visitor.

* Create Event union while adding to it.

* Add bool to typemappings.

* Add method descriptions and remove output type mappings.

* Add storage mode.

* Add URL support and a flattening script.

* Fix parameter for flattening in README.md.

* Add setup and references to README.md.

Co-authored-by: prathamesh <prathamesh.musale0@gmail.com>

packages/codegen/README.md

@@ -0,0 +1,83 @@
+# Code Generator
+
+## Setup
+
+* Run the following command to install required packages:
+
+  ```bash
+  yarn
+  ```
+
+## Run
+
+* Run the following command to generate a flattened contract file from a contract file:
+
+  ```bash
+  yarn codegen:flatten <input-file-path> [output-dir]
+  ```
+
+    * `input-file-path`: Input contract file path (absolute) (required). Note: Currently, relative path doesn't work.
+    * `output-dir`: Directory to store the flattened contract output file (default: `./out`).
+
+  Example:
+  
+  ```bash
+  yarn codegen:flatten ~/watcher-ts/node_modules/@openzeppelin/contracts/token/ERC20/ERC20.sol ./flattened
+  ```
+
+  This will generate file `ERC20_flat.sol` in `./flattened`.
+
+* Run the following command to generate schema from a contract file:
+
+  ```bash
+  yarn codegen:gql --input-file <input-file-path> --output-file [output-file-path] --mode [eth_call | storage]
+  ```
+
+    * `input-file`: Input contract (must be a flattened contract) file path or an URL (required).
+    * `output-file`: Schema output file path (logs output using `stdout` if not provided).
+    * `mode`: Contract variables access mode (default: `storage`).
+
+  Examples:
+  
+  ```bash
+  yarn codegen:gql --input-file ./test/examples/contracts/ERC20-flat.sol --output-file ./ERC20-schema.gql --mode eth_call
+  ```
+
+  ```bash
+  yarn codegen:gql --input-file https://git.io/Jupci --output-file ./ERC721-schema.gql --mode storage
+  ```
+
+## Demo
+
+* Install required packages:
+
+  ```bash
+  yarn
+  ```
+
+* Flatten a contract file:
+
+  ```bash
+  # Note: Currently, relative path for input-file-path doesn't work. Use absolute path.
+  yarn codegen:flatten ~/watcher-ts/node_modules/@openzeppelin/contracts/token/ERC20/ERC20.sol ./flattened
+  ```
+
+
+* Generate schema from the flattened contract file:
+  
+  ```bash
+  yarn codegen:gql --input-file ./flattened/ERC20_flat.sol --output-file ./ERC20-schema.gql --mode storage
+  ```
+
+* Generate schema from the flattened contract file from an URL:
+  
+  ```bash
+  yarn codegen:gql --input-file https://git.io/Jupci --output-file ./ERC721-schema.gql --mode eth_call
+  ```
+
+## References
+
+* [ERC20 schema generation (eth_call mode).](https://git.io/JuhN2)
+* [ERC20 schema generation (storage mode).](https://git.io/JuhNr)
+* [ERC721 schema generation (eth_call mode).](https://git.io/JuhNK)
+* [ERC721 schema generation (storage mode).](https://git.io/JuhN1)