Fix perturbation assay names with invalid characters

[rootze]

.sanitize_perturbation_name() is for creating perturbation assays. It takes the user-provided name, checks that it is valid input, converts it to a safe R/Seurat-style name with make.names(), and warns if the name changed. It is used near the start of

ModulePerturbation()
TFPerturbation()
CustomPerturbation()

.resolve_perturbation_assay_name() is for finding perturbation assays that may already exist.
It checks: Does the exact name exist in seurat_obj@assays? If not, does the sanitized version exist? If neither exists, return the original name so the normal error message can say it was not found.