Update custom property output methodology

[sebnitu]

Problem

With the introduction of custom properties (CSS variables), there hasn't been established a consistent way of including global custom properties that doesn't lead to output duplication.

Solution

This PR refactors how custom properties are organized and output. Instead of outputting them in the core module, the core module instead exposes root scss and css output files for manual inclusion. The global vrembem package now also includes all root custom properties by default.

SCSS Example

// Include specific custom properties
@use "@vrembem/core/src/scss/root/prefix";
@use "@vrembem/core/src/scss/root/breakpoints";

// Include all custom properties
@use "@vrembem/core/src/scss/root/root";

CDN Example

<!-- Include specific custom properties -->
<link rel="stylesheet" href="https://unpkg.com/@vrembem/core/dist/prefix.css">
<link rel="stylesheet" href="https://unpkg.com/@vrembem/core/dist/breakpoints.css">

<!-- Include all custom properties -->
<link rel="stylesheet" href="https://unpkg.com/@vrembem/core/dist/root.css">

With this update and in preparation for future custom properties, this PR also updates the custom property prefix default to "vb-". That means all vrembem CSS variables should be prefixed with vb-, e.g: --vb-breakpoint-md: 760px;.

This PR fixes #854