feat: inttroduce the process() method

This commit introduces a new method (`process()`) that will invoke compilation or transpilation depending on the file extension. This should reduce the amount of code that we need to write in each gulp build file, ex.:
https://github.com/PK1A/hsp-hello-gulp/commit/80656fcfab7cca8948f7386e34d6037af2f847f1