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/highlighted/Plaintext/plaintext.txt

@@ -175,4 +175,4 @@
 \u{ad}␊
 \u{ae}␊
 ␊
-Here's·a·line·with·multiple·characters.
+Here's·a·line·with·multiple·characters.␊