Parsing key with leading colon

I'm attempting to replace `yaml-cpp` with rapidyaml in my project. 

An example document is like 
```
:foo:
  :bar: a
  :barbar: b
  :barbarbar: c
```

Although `yaml-cpp` is able to parse with the leading colon character, rapidyaml is unable to handle this
```
1:1:ERROR: parse error
1:1: :foo:  (size=5)
     ^~~~~  (cols 1-6)
```