Make `.nvmrc` more versatile/modular #1134

Job	Run time
shellcheck_matrix (bash, nvm.sh)	40s
shellcheck_matrix (sh, nvm.sh)	45s
shellcheck_matrix (dash, nvm.sh)	47s
shellcheck_matrix (ksh, nvm.sh)	35s
shellcheck_matrix (bash, install.sh)	27s
shellcheck_matrix (bash, bash_completion)	25s
shellcheck_matrix (bash, nvm-exec)	28s
shellcheck	8s
	4m 15s

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