typings: add missing binding function `writeFileUtf8()`

writefileutf8() binding function was missing in fs.d.ts.

It is defined as follows: https://github.com/nodejs/node/blob/3c50297e9e68553c4ee31e7f3fd677c8360a11f0/src/node_file.cc#L3705 https://github.com/nodejs/node/blob/3c50297e9e68553c4ee31e7f3fd677c8360a11f0/typings/internalBinding/fs.d.ts#L240-L241