Add command to validate configuration files

Summary:
Currently, the command validates JSON syntax,
using the own PHP parser instead to rely on jsonlint,
to avoid to depends of Node.JS for one command.

Fixes T1177.

Test Plan:
New tests check command output is expected when:

• all the files are valid
• there is a general syntax error
• there is a specific syntax error

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1177

Differential Revision: https://devcentral.nasqueron.org/D949