Fix those damn placeholder conditions

[NicoNekoDev]

Firstly, in the wiki (https://plugins.auxilor.io/effects/all-conditions/placeholder_greater_than) it says "greater than or equal" for placeholder_higher_than condition, which it's not actually the case as the operator is > and not >=.
Secondly, it should be a mathematical evaluation, not a formatted string followed by a conversion to double. Probably this is why in most cases, those 2 conditions don't work accordingly with placeholders, if any space, or character, or whatever is present.
Thirdly, allow placeholders to be evaluated in both placeholder and value arguments.

[NicoNekoDev]

Please also consider adding an optional argument or_equals instead of forcing to be equals too (for both conditions).