Better YAML parser

[azat]

Right now ClickHouse parser does not recognizes that the following is invalid:

----
# foo
bar: 1

And it does not throw any error and simply does not interpret bar: 1 at all:

yaml-cpp

$ util/parse /tmp/doc-start2.yaml
----

yq

$ yq /tmp/doc-start2.yaml
----
Error: bad file '/tmp/doc-start2.yaml': yaml: line 2: did not find expected <document start>

I did a brief review of existing YAML libraries:

• libyaml - Merge keys appear not to be supported yaml/libyaml#168
  • does not support merge operator
  • parser API emit alias events
• https://github.com/pantoniou/libfyaml
  • not popular enough
• https://github.com/biojppm/rapidyaml
  • merges keys incorrectly - Merge key order biojppm/rapidyaml#484
  • code is complex
  • c4core dependency
• yaml-cpp
  • document start/end parsing is wrong - Forbid incorrect document start (---) jbeder/yaml-cpp#1333

TL;DR; - the issue in yaml-cpp should be fixed