feat(useSpeechSynthesis): add stop function

[zwkkwz1]

Warning: ⚠️ Slowing down new functions

Due to the growing audience of VueUse, we received a huge amount of feature requests and pull requests. It's become harder and harder and recently a bit beyond our capacity to maintain the project. In the near future, we could like slowing down on accepting new features and prioritize the stability and quality of existing functions. New functions to VueUse may not be accpected. If you come up some new ideas, we advice you to have them in your codebase first instead of proposing to VueUse. You may iterate them a few time and see how them suite your needs and how them can be generalized. If you really believe they are useful to the community, you can create PR with your usercases, we are still happy to hear and discuss. Thank you for your understanding.

Description

after starting to speak, clicking stop will stop the sound, but the speak button will not be enabled again.(fixes #2764)

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (fixes #2764).
• Ideally, include relevant tests that fail without this PR but pass with it.

[antfu]

Suggested change

	const isStop = () => {
	const stop = () => {

It should be a verb... right?

[zwkkwz1]

right..