quote:false lets consumers opt out of the default JSON-quoted env-file
output; sortKeys lets writeEnvFileObject take a custom key order.
writeProcessEnvObject's ENV_FILE_VALUE handling now respects
onlyMissing instead of always overwriting an already-set ENV_FILE.
Also adds readProcessEnvFlag/readProcessEnvNumber/
readOptionalNonNegativeEnvNumber, pure env-value parsing helpers with
no prior equivalent here.