Don't add artificial newline to last line if --style=plain

This fixes #1438.

Note however, that using a pager such as less will add a newline itself.
So to actually not print a newline for such files, you need to either
disable paging:

  bat --style=plain --paging=never no-newline-at-end-of-file.txt

or use a "pager" that does not add a newline:

  bat --style=plain --pager=cat no-newline-at-end-of-file.txt

Note that we also update syntax tests file since a bunch of them had
missing newlines on the last lines.

tests/syntax-tests/source/Git Attributes/example.gitattributes

@@ -13,4 +13,4 @@
 *.patch    -text
 
 .gitattributes linguist-language=gitattributes
-.gitkeep       export-ignore
+.gitkeep       export-ignore