[Fix] `nvm_normalize_lts`: switch from expr to case #1118

Job	Run time
shellcheck_matrix (bash, nvm.sh)	46s
shellcheck_matrix (sh, nvm.sh)	55s
shellcheck_matrix (dash, nvm.sh)	53s
shellcheck_matrix (ksh, nvm.sh)	58s
shellcheck_matrix (bash, install.sh)	32s
shellcheck_matrix (bash, bash_completion)	27s
shellcheck_matrix (bash, nvm-exec)	22s
shellcheck	0s
	4m 53s

Provide feedback