Implement indented here documents <<~EOF like Perl does

### Expected behavior

https://perldoc.perl.org/perlop#Indented-Here-docs

```
template {
data = <<~CONF
F2
CONF
destination = "custom/walk-inside.com.conf"
}
```
Should generate

```
F2
```

### Actual behavior

It generates
```
F2
```
Leading spaces are awkward. I could write without them:
```
template {
data = <

Start by locating the here-document parsing and rendering entry points in the Go repository, then compare the issue's <<~CONF example with the existing <

Written by the indexing model from the issue text.