Added Japanese localization for sharing to the Mastodon.

[sussan0416]

• Translated and enabled the previously commented-out mastodon: "Toot on Mastodon" in i18n/ja.yaml.
• Ran npm run example and confirmed that the Tooltip is localized in Japanese, as shown in the image below.
• If there are no problems, I would appreciate it if you could merge it into the dev branch.

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[sussan0416]

Could you please add the 'i18n' label?
I'm unable to edit labels after the pull request has been opened. 🙏

[jpanther]

Thanks, the labeller had a config issue that has since been fixed. I'm happy to include this translation, thanks for the updated string. I'll merge it in now.