Fix File.Stream Enumerable.count for files without trailing newline

[pnezis]

The optimized count implementation was counting newline characters instead of actual lines, returning incorrect results for files that don't end with a newline (which is common for source code files).

For example, a file with content "line1\nline2\nline3" (3 lines) was returning 2 instead of 3.

The fix tracks the last byte read and adds 1 at EOF if the file has content but doesn't end with a newline.

You can reproduce it with:

iex> File.write!("/tmp/test.txt", "foo")
iex> stream = File.stream!("/tmp/test.txt")
iex> Enum.count(stream)
0

[josevalim]

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