Move assets folder into src/assets

[Raruto]

Related to: #74, #76, #77, #78, #79, #80, #81, #83

Asset files are also compiled, so it's best to put them inside the src folder as well. However, in the long run, it is always better to adopt a less nested folder structure than the current one (v3.4), further complexity could be easily handled by simply creating and including external javascript libraries via npm.

Reference:

• https://vueschool.io/articles/vuejs-tutorials/how-to-structure-a-large-scale-vue-js-application/