feat: Add concat function to README

This commit adds the `concat` function to the list of functions in the
README file. This function is used to concatenate strings.

Author: ryoppippi

README.md

@@ -26,6 +26,7 @@ A collection of type-safe string functions written in pure TypeScript.
 - [`lowercase`](src/lowercase.ts) - Convert a string to lowercase.
 - [`split`](src/split.ts) - Split a string into substrings using the specified
   separator and return them as an array.
+- [`concat`](src/concat.ts) - Concatenate strings.
 
 ## Related Works