fix: add class content to fix missing underline on button component (m…

…icrosoft#4226)

# Description

Added `class="content"` to content span in button template.
In both the FAST and FluentUI lightweight button, the underline on hover and focus is missing. The style for the underline is using `content::before` selector, so adding the content class fixes the issue.
This should fix both FAST and FluentUI.


## Motivation & context

<!--- What problem does this change solve? -->
microsoft#4201 for FAST
#16271 for fluentui

## Issue type checklist

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**Is this a breaking change?**
- [ ] This change causes current functionality to break.

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**Adding or modifying component(s) in `@microsoft/fast-components` checklist**

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packages/web-components/fast-foundation/src/button/button.template.ts

@@ -45,7 +45,7 @@ export const ButtonTemplate = html<Button>`
         ${ref("control")}
     >
         ${startTemplate}
-        <span part="content">
+        <span class="content" part="content">
             <slot ${slotted("defaultSlottedContent")}></slot>
         </span>
         ${endTemplate}