perf(plugin-back-to-top): optimize backToTop icons

[pengzhanbo]

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• [ ] Bug fix
• [x] New feature
• [ ] Other

Description

Replace the current backToTop icon and adjust its size.

Screenshots

Before

[coveralls]

Pull Request Test Coverage Report for Build 9045487184

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 50.304%

---

Totals
Change from base Build 9041934374:	0.0%
Covered Lines:	698
Relevant Lines:	1255

---

💛 - Coveralls