Congifuration files now support multi-line string values with indentation.

Introduced the `"""~` and `~"""` to quote indented lines. For example:

```
rule_xlu4 {
  sql = """~
    SELECT
      *
    FROM
      "t/#"
  ~"""
}
```
See [HOCON 0.42.0](https://github.com/emqx/hocon/releases/tag/0.42.0) release note for more dtails.