chore: separate grid column component into its own directory

[melaniebmn]

Summary | Résumé

Move the grid column component to its own directory.

This PR separates the grid-col component into its own dedicated directory. The goal is to provide better organization and enable the creation of individual README for both the grid and the grid-col components.

How to test

The new grid README can be viewed here and the new grid-col README can be viewed here.

Both components work the same as before. They can be tested by using this example code:

<gcds-grid columns-desktop="1fr 1fr 1fr 1fr" columns-tablet="1fr 1fr" columns="1fr">
    <p>Example content</p>
    <p>Example content</p>
    <p>Example content</p>
    <p>Example content</p>
</gcds-grid>

<gcds-grid>
    <gcds-grid-col tablet="3" desktop="3">Example content for grid column.</gcds-grid-col>
    <gcds-grid-col tablet="3" desktop="3">Example content for grid column.</gcds-grid-col>
    <gcds-grid-col tablet="3" desktop="3">Example content for grid column.</gcds-grid-col>
    <gcds-grid-col tablet="3" desktop="3">Example content for grid column.</gcds-grid-col>
</gcds-grid>

[ethanWallace]

LGTM

[daine]

LGTM, tested locally by using gcds-grid-col within a gcds-grid as usual and it works the same