Make Manage Reusable blocks match similar links

[dsas]

What?

The Manage Reusable blocks link inside the reusable blocks inserter should match other similar links such as 'Explore all patterns' on the pattern inserter.

Why?

User interface consistency. Fixes #45155

How?

This changes a HTML <a> to a <Button variant=secondary /> along with some CSS changes

Testing Instructions

Add a reusable block if you haven't already:

1. Open Gutenberg post editor
2. Insert any block
3. Press the three dots menu of the block and choose 'Create reusable block'
4. Follow the prompts

Test the change:

1. Still in the editor, open the inserter (big blue plus at the top left)
2. In the inserter sidebar, click Patterns and scroll to the bottom, look at the 'Explore all patterns' button
3. Click 'Reusable blocks' in the inserter sidebar and scroll to the bottom, look at the 'Manage Reusable blocks' button. They should look the same
4. Click 'Manage Reusable blocks' button it should take you to /wp-admin/edit.php?post_type=wp_block

Screenshots or screencast

Before	After

[codesandbox]

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[vcanales]

LGTM 👍

[jameskoster]

I should have mentioned this on the issue itself, but in addition to the appearance of the button, the positioning should match as well. Would it be possible to add this change in a follow-up?

Edit: nvm, I see Nik has a PR open merged :D