add missing part attribute for tabpanel in tabs component (#6543)

<!---
Thanks for filing a pull request 😄 ! Before you submit, please read the following:

Search open/closed issues before submitting. Someone may have pushed the same thing before!

Provide a summary of your changes in the title field above.
-->

# Pull Request

## 📖 Description

<!---
Provide some background and a description of your work.
What problem does this change solve?
Is this a breaking change, chore, fix, feature, etc?
-->

### 🎫 Issues

<!---
* List and link relevant issues here.
-->

## 👩‍💻 Reviewer Notes

<!---
Provide some notes for reviewers to help them provide targeted feedback and testing.

Do you recommend a smoke test for this PR? What steps should be followed?
Are there particular areas of the code the reviewer should focus on?
-->

## 📑 Test Plan

<!---
Please provide a summary of the tests affected by this work and any unique strategies employed in testing the features/fixes.
-->

## ✅ Checklist

### General

<!--- Review the list and put an x in the boxes that apply. -->

- [ ] I have included a change request file using `$ yarn change`
- [ ] I have added tests for my changes.
- [ ] I have tested my changes.
- [ ] I have updated the project documentation to reflect my changes.
- [ ] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/master/CONTRIBUTING.md) documentation and followed the [standards](/docs/community/code-of-conduct/#our-standards) for this project.

### Component-specific

<!--- Review the list and put an x in the boxes that apply. -->
<!--- Remove this section if not applicable. -->

- [ ] I have added a new component
- [ ] I have modified an existing component
- [ ] I have updated the [definition file](https://github.com/microsoft/fast/blob/master/packages/web-components/fast-components/CONTRIBUTING.md#definition)
- [ ] I have updated the [configuration file](https://github.com/microsoft/fast/blob/master/packages/web-components/fast-components/CONTRIBUTING.md#configuration)

## ⏭ Next Steps

<!---
If there is relevant follow-up work to this PR, please list any existing issues or provide brief descriptions of what you would like to do next.
-->

packages/web-components/fast-foundation/src/tabs/tabs.ts

@@ -345,7 +345,8 @@ export class Tabs extends FoundationElement {
         const prev: number = this.activeIndicatorRef[offsetProperty];
         this.activeIndicatorRef.style[gridProperty] = `${this.activeTabIndex + 1}`;
         const next: number = this.activeIndicatorRef[offsetProperty];
-        this.activeIndicatorRef.style[gridProperty] = `${this.prevActiveTabIndex + 1}`;
+        const gridPropertyValue = this.prevActiveTabIndex === -1 ? 0 : this.prevActiveTabIndex;
+        this.activeIndicatorRef.style[gridProperty] = `${gridPropertyValue + 1}`;
         const dif: number = next - prev;
         this.activeIndicatorRef.style.transform = `${translateProperty}(${dif}px)`;
         this.activeIndicatorRef.classList.add("activeIndicatorTransition");