fs: move SyncWriteStream to internal/fs
Checklist
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tests and code linting passes -
documentation is changed or added -
the commit message follows commit guidelines
Affected core subsystem(s)
fs
Description of change
Move the internal SyncWriteStream class to internal/fs. This pulls it off require('fs') without a deprecation. It was always intended to be private, says that it's private in the source and tells people not to use it.
Update the impl a bit while we're at it to use class instead of util.extends