Extend static_show() escaping to any illegal identifier via var"".

Previously, it was only escaping names with a `#` in them, but now it
will escape all names that are illegal identifiers.

Before:
```julia
julia> struct var"a%b+c" x::Int end

julia> println(static_shown(var"a%b+c"))
Main.a%b+c

julia> println(static_shown(var"a%b+c"(1)))
Main.a%b+c(x=1)
```
After:
```julia
julia> struct var"a%b+c" x::Int end

julia> println(static_shown(var"a%b+c"))
Main.var"a%b+c"

julia> println(static_shown(var"a%b+c"(1)))
Main.var"a%b+c"(x=1)
```

src/rtutils.c

@@ -625,7 +625,10 @@ static size_t jl_static_show_x_sym_escaped(JL_STREAM *out, jl_sym_t *name) JL_NO
     size_t n = 0;
 
     char *sn = jl_symbol_name(name);
-    int hidden = strchr(sn, '#') != 0;
+    int hidden = 0;
+    if (!jl_is_identifier(sn) || jl_is_operator(sn)) {
+        hidden = 1;
+    }
 
     if (hidden) {
         n += jl_printf(out, "var\"");

test/show.jl

@@ -1339,6 +1339,7 @@ end
 # PR #38049
 struct var"#X#" end
 var"#f#"() = 2
+struct var"%X%" end  # Invalid name without '#'
 
 # (Just to make this test more sustainable,) we don't necesssarily need to test the exact
 # output format, just ensure that it prints at least the parts we expect:
@@ -1350,6 +1351,8 @@ var"#f#"() = 2
     @testset for v in (
             var"#X#",
             var"#X#"(),
+            var"%X%",
+            var"%X%"(),
             Vector,
             Vector{<:Any},
             Vector{var"#X#"},