src/cpp-common: add `bt2c::TextLoc` class

src/cpp-common: add `bt2c::TextLoc` class

This patch adds the `bt2c::TextLoc` class, defined in `text-loc.hpp` and
implemented in `text-loc.cpp`.

A text location is a location within some text: an offset (in bytes), a
line number, and a column number.

This is part of an effort to implement a JSON parser to
support CTF2‑SPEC‑2.0 [1].

[1]: https://diamon.org/ctf/CTF2-SPEC-2.0.html

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ice9cab35041f1637d220deb6af8f52dbfc8cd290
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7408
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12678