Adjust .editorconfig

Adjust .editorconfig

 - Move current indent_style and tab_width to the [*] section, so it
   applies by default to all files (.c, .h,, .l, .y, Makefile).
 - Add indent_size to [*]
 - Add handling of .hpp and .cpp file, which use indent of 4 spaces and
   no tabs, like .py files.

Change-Id: Ifc8028eb7ba00dfcd2d3904d3301b66bdfa28d79
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7097
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>