Support the import statement of molecule for the codegen

[homura]

The first version of the codegen doesn't support the import statement of Molecule yet. To deal with the cases of importing other schemas, the codegen CLI also needs to

1. resolve all import schema items
2. remove the intersection with prepend
3. append them to the prepend

Originally posted by @homura in #635 (comment)

Note:

The parser of Molecule written in JS doesn't support the import statement, therefore, it is better to handle the import statement separately. The instructions for handling the import feature could be

1. resolve the items from the import statement
2. convert the exported items into ESM syntax and append these items into the prepend
3. erase the import clause from the schema when calling the codegen method
4. generate code recursively and name outputs the same with schema's

For example, now we have 2 schema files, base and character

// base.mol

array RGB [byte;3];
vector UTF8String <byte>;

// character.mol

import base;

table Character {
  hair_color: RGB, 
  name: UTF8String,
}

After the codegen, it should be

// base.ts
export const RGB = array(byte, 3);
export const UTF8String = vector(byte);

// character.ts

import { RGB, UTF8String } from './base'

export const Character = table({
  hair_color: RGB,
  name: UTF8String,
}, ['hair_color', 'name'])