fish_indent: Allow escaped newlines only for certain things

Things like

```fish
\
echo foo
```

or

```fish
echo foo; \
echo bar
```

are a formatting blunder and should be handled.

This makes it so the escaped newline is removed, and the
semicolon/token_type_end handling will then put the statements on
different lines.

One case this doesn't handle brilliantly is an escaped newline after a
pipe:

```fish
echo foo | \
cat
```

is turned into

```fish
echo foo | cat
```

which here works great, but in long pipelines can cause issues.

Pipes at the end of the line cause fish to continue parsing on the
next line, so this can just be written as

```fish
echo foo |
cat
```

for now.

tests/indent.out

@@ -107,8 +107,7 @@ while true
     builtin yes
 end
 
-alpha | \
-    beta
+alpha | beta
 
 gamma | \
     # comment3