[feature] Implement more powerful fix methods

[HerringtonDarkholme]

This is an epic issue to track more powerful fix methods

• Change fix option from string to Fixer
• Implement selection expander
• Implement prepend/append #1105
• update json schema
• update doc

[HerringtonDarkholme]

Expand node selection range

⭐ Suggestion

ast-grep rule can only fix one target node at one time. This works fine for function statement/calls but it has always been problematic for list-item like items in an array, key-value pairs in a dictionary. We cannot delete an item because we also need to delete the surrounding comma.

Similarly, the problem also applies to a statement if it has a semicolon.

💻 Use Cases

Add the expandTarget field to rule.

language: python
rule:
  kind: pair
  has:
   find: regex
expandTarget:
  forward: 1 # forward one byte?
  forward: 
    regex: , # or a rule object
fix: '' # delete

related: #746

The expand struct

expandForward:  # relation
  regex: ','
  stopBy: end

[HerringtonDarkholme]

Add prepend/append statements in fix

⭐ Suggestion

It is usually desirable to prepend/append extra content to a file during a code fix.
For example, setting up a top-level variable, importing a function and etc.

💻 Use Cases

Suppose we want to extract some repetitive code.

const result = () => {
  doSomeRepetitiveCode()
}

to

import { importedUtil } from 'utils' // note this line
const result = importedUtil()

The first line should import the extracted util function from another file.

---

Change console.log to logger.log

console.log('message')

to

import logger from 'logger'
logger.log('message')