Feature/target specific tasks

[ruben-arts]

Adding the ability to add target (platform) specific tasks. The idea is that a task is cross platform by definition but sometimes you need extra parameters on different platforms for instance:

[tasks]
build = {cmd="colcon build --build-base .pixi/build --install-base .pixi/install"}
test = {cmd="colcon test --build-base .pixi/build --install-base .pixi/install"}

[target.win-64.tasks]
build = {cmd="colcon build --merge-install --build-base .pixi/build --install-base .pixi/install"}
test = {cmd="colcon test --merge-install --build-base .pixi/build --install-base .pixi/install"}

This pr allows for that.

closes #192

[wolfv]

Looks good to me!