After !6954 got merged, it has become easier for tools to get
full stack-traces for runtime traces of a CMake program. The trace
information already included in the JSON objects (line number, source
file path) allows tools that display these stack traces to print the
CMake source code associated to them. However, CMake commands may
spawn multiple lines, and the JSON information associated to a trace
only contains the line in which the command started, but not the one
in which it ended. If tools want to print stack traces along the
relevant source code, and they want to print the whole command
associated to the stack frame, they will have to implement their own
CMake language parser to know where the command ends.

In order to simplify the life of those who want to write tooling for
CMake, this commit adds a `line_end` field to the json-v1 trace
format. If a given command spans multiple lines, the `line_end` field
will contain the line of the last line spanned by the command (that of
the closing parenthesis associated to the command).