Tensura: Skill Obtain

Added configurable drop chances for individual skills.

Previously, skills assigned to the same entity shared a single drop chance. For example:
"tensura:slime|tensura:self_regeneration,tensura:charm|1.0"
This forced all listed skills to use the same chance.

With the new format, each skill can now have its own independent chance:
"tensura:slime|tensura:self_regeneration,mysticism:crasher,tensura:tuner|1.0,0.01,0.5"

Updated config handling for better flexibility.
Existing configurations remain compatible. All previously defined skills now default to a 1.0 chance, making it easier to fine-tune individual chance without needing to rewrite entries.