ToC block: Update link icon

[t-hamano]

Closes #653

Use the same SVG data as the link icons exposed in the icon library for the heading link icons.

Before	After

Testing Instructions

• Activate Twenty Twenty-Five
• Appearance > Editor > Template > Single Post
• Insert a ToC block outside the Content block. Otherwise, the the_content hook won't work properly.
• Create a new post and insert multiple headings.
• Open the post on the frontend and check the icon.

[ryelle]

This icon is a big improvement, thanks! Just one note about an unrelated change.