feat: ensure there's at least 1 char in non-empty string

This is useful e.g. when trying to leverage a non-empty string for accessing its first character. We can say there's always at least 1 char in non-empty string so this allows us to do `s[0]` with ts option `noUncheckedIndexedAccess: true`.

source/types.ts

@@ -73,3 +73,5 @@ export type NodeStream = {
 } & NodeJS.EventEmitter;
 
 export type Predicate = (value: unknown) => boolean;
+
+export type NonEmptyString = string & {0: string};