fish_indent: indent line continuations

For example:

    cmd \
        arg

Fixes one case from #7252

src/fish_indent.cpp

@@ -400,6 +400,10 @@ struct pretty_printer_t {
                     output.append(L" ");
                 }
                 output.append(L"\\\n");
+                // Indent the line continuation (#7252).
+                current_indent += 1;
+                emit_space_or_indent();
+                current_indent -= 1;
             }
         }
 

tests/checks/indent.fish

@@ -248,6 +248,25 @@ end
 #CHECK: {{    }}# comment
 #CHECK: end
 
+echo -n '
+cmd \\
+continuation
+' | $fish_indent
+#CHECK: cmd \
+#CHECK: {{    }}continuation
+
+echo -n '
+begin
+cmd \
+continuation
+end
+' | $fish_indent
+#CHECK: begin
+#CHECK: {{    }}cmd \
+#CHECK: {{    }}{{    }}continuation
+#CHECK: end
+
+
 echo -n '
 i\
 f true