improve invalid tuple errors

[xdBronch]

this does 2 things

1. add a note when a tuple field has a name, fixes Unintuitive error for omitting field type in struct #23045
   before:

a.zig:1:20: error: tuple field has a name
const S = struct { a: usize = 1, b = 2, c: isize = 3, d: []const u8 };
                   ^

after:

a.zig:1:20: error: tuple field has a name
const S = struct { a: usize = 1, b = 2, c: isize = 3, d: []const u8 };
                   ^
a.zig:1:34: note: tuple field here
const S = struct { a: usize = 1, b = 2, c: isize = 3, d: []const u8 };
                                 ^~~~~

2. fix the note when a decl is encountered in a tuple but the first field also has a name

const S2 = struct {
    const a = 0;
    b: u8 = 0,
    u8,
};

before:

a.zig:2:5: error: tuple declarations cannot contain declarations
    const a = 0;
    ^~~~~~~~~~~
a.zig:3:5: note: tuple field here
    b: u8 = 0,
    ^~~~~~~~~

after:

a.zig:2:5: error: tuple declarations cannot contain declarations
    const a = 0;
    ^~~~~~~~~~~
a.zig:4:5: note: tuple field here
    u8,
    ^~

[rohlem]

seems to be an alternative to #20762
(I personally prefer the other PR, because struct types are more common in my experience, therefore it makes sense to treat them as the default.
Having a note point at the tuple field is a handy addition though, maybe we could have both?)

[xdBronch]

thanks, yeah i probably would prefer what the other pr is doing there, i just did the simplest fix rather than changing the error. i do believe that pr will need to be redone since tuples have been reworked though, not sure how much more difficult it might be