[core] getMobHP binding for mobskills

[UmeboshiXI]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Adds binding getMobHP binding for use in mobskill scripts. This retrieves the mob's HP when it firsts attempts to use the skill(Beginning of prepare time).

Use case: Many suicide skills use the mobs current HP(at the time it first started casting) for damage calculations.

Steps to test these changes

Currently not used, just ground work for upcoming PRs. You can test with prints.

• Add a print in a onMobWeaponSkill(within a mobskill script) with the new binding: skill:getMobHP()
• Force the mob to use the skill you edited with !tp 3000 when engaged or !exec target:useMobAbility(AbilityID, player)
• See print returns mobs HP as it was on skill prepare rather than at time of execution.

[zach2good]

Please change new functions to use trailing return syntax, and be const if possible:

auto CLuaMobSkill::getMobHP() const -> int32
{
  ....

Same applies in the header