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.

CHANGELOG.md

@@ -5,6 +5,8 @@
 
 ## Bugfixes
 
+- If the last line doesn't end with a newline character, don't add it if `--style=plain`, see #1438 (@Enselic)
+
 ## Other
 
 ## Syntaxes