tasks: list tasks with proper alignment

[kraptor]

This is a small QoL patch.

Currently, nimble lists tasks without proper alignment between the name and the description of the task:

$ nimble tasks
build_debug        Build debug version
build_release        Build release version
build_profiler        Build with profiler
build_all        Build all versions
clean        Clean all build files

This patch adds the ability to collect the max length of the tasks names (with a minimum of 8 characters), then render descriptions properly aligned:

$ ./nimble tasks
build_debug     Build debug version
build_release   Build release version
build_profiler  Build with profiler
build_all       Build all versions
clean           Clean all build files

I tried to keep the same code style as the code aroudn. Please, let me know if there is anything else you'd like to improve/change.

NOTE: this change may potentially break scripts dealing with nimble tasks output, as it does inject two spaces between task name and task description, instead of a bunch of spaces like before.

The following tests did not pass on master before, not sure if because of my installation of Nim (using 1.5.1-latest) or my system:

• nimscript evaluation error message

[kraptor]

Ok, I see what is failing... let me fix printSeqIfLen template...

[kraptor]

I squashed the changes for easy review @dom96, everything seems ok now.