Add index to LineResult

[typed-sigterm]

Identifying the start index of an issue. (working for #233)

let raw = indoc! { r###"
Hello world

```ts
// hello世界
const a = "string字符串";
```

### 外部test

Second line

```rb
class User
    # 查找user
    def find
    end
end
```
"###};

let expected = indoc! { r###"
{
  "filepath": "md",
  "lines": [
    {
      "l": 4,
      "c": 1,
      "index": 19,
      "new": "// hello 世界",
      "old": "// hello世界",
      "severity": 1
    },
    {
      "l": 5,
      "c": 11,
      "index": 40,
      "new": "\"string 字符串\"",
      "old": "\"string字符串\"",
      "severity": 1
    },
    {
      "l": 8,
      "c": 5,
      "index": 62,
      "new": "外部 test",
      "old": "外部test",
      "severity": 1
    },
    {
      "l": 14,
      "c": 5,
      "index": 104,
      "new": "# 查找 user",
      "old": "# 查找user",
      "severity": 1
    }
  ],
  "error": ""
}
"###};

let result = lint_for(raw, "md");
assert_json_eq!(expected, result.to_json_pretty());