Refactor tokens file/folder structure to support additional theme variants

[adamstankiewicz]

Description

The current structure of the tokens directory contains a top-level src folder containing all the tokens for the design system (e.g., global, alias, component). As described in #2014, these tokens are largely a combination of core tokens and tokens representing the values for the default "light" theme variant.

To be forward thinking to support a future dark mode (i.e., an additional theme variant), let's extract the core tokens out from the theme-specific tokens. I might recommend a folder/file structure similar to:

tokens/src/core/
tokens/src/themes/light/
tokens/src/themes/dark/ # in the future

By accomplishing this issue, consuming applications should be able to load the core Paragon styles independently from any theme-variant CSS variables. For example:

<link rel="stylesheet" href="/dark.css" media="(prefers-color-scheme: dark)" />
<link
  rel="stylesheet"
  href="/light.css"
  media="(prefers-color-scheme: light)"
/>
<link rel="stylesheet" href="/core.css" />

Acceptance Criteria

• Ensure the global, alias, and component tokens defined in ./tokens/src/core/ are not specific to a particular theme variant (i.e., light or dark).
• Ensure the global, alias, and component tokens defined in ./tokens/src/themes/light/ are only related to style properties relevant to a light theme variant (most likely color tokens).

[PKulkoRaccoonGang]

PR: #2069