Correct syntax highlighting for variables spanning multiple lines

A variable may be broken across multiple lines with a backslash, for
example:

     > echo $FISH_\
       VERSION

Teach syntax highlighting about this line breaking. Fixes #8444

src/fish_tests.cpp

@@ -5525,6 +5525,13 @@ static void test_highlighting() {
         {L"=", highlight_role_t::operat, ns},
     });
 
+    // Highlighting works across escaped line breaks (#8444).
+    highlight_tests.push_back({
+        {L"echo", highlight_role_t::command},
+        {L"$FISH_\\\n", highlight_role_t::operat},
+        {L"VERSION", highlight_role_t::operat, ns},
+    });
+
     auto &vars = parser_t::principal_parser().vars();
     // Verify variables and wildcards in commands using /bin/cat.
     vars.set(L"VARIABLE_IN_COMMAND", ENV_LOCAL, {L"a"});

src/highlight.cpp

@@ -489,8 +489,16 @@ static size_t color_variable(const wchar_t *in, size_t in_len,
     }
 
     // Handle a sequence of variable characters.
-    while (valid_var_name_char(in[idx])) {
+    // It may contain an escaped newline - see #8444.
-        colors[idx++] = highlight_role_t::operat;
+    for (;;) {
+        if (valid_var_name_char(in[idx])) {
+            colors[idx++] = highlight_role_t::operat;
+        } else if (in[idx] == L'\\' && in[idx + 1] == L'\n') {
+            colors[idx++] = highlight_role_t::operat;
+            colors[idx++] = highlight_role_t::operat;
+        } else {
+            break;
+        }
     }
 
     // Handle a slice, up to dollar_count of them. Note that we currently don't do any validation of