What syntax should struct patterns use?

Summary of issue:

When a struct pattern has a default, it can look like this: {.a = b: T = c}. The two =s are problematic and visually confusing. In #7530 we decided we wanted to replace the first = with something else. We need to decide what replaces it.

Details:

The = here is problematic both because it conflicts with = for defaults, and also because it's the only context in Carbon where we use = where we don't (conditionally or unconditionally) assign or bind the value on the right to the thing on the left. This = signifies a nested match.

Candidate alternatives suggested so far:

• {.a: b}: unify struct literal and struct type literal syntax; {.a: b} is always a struct value, but has an implicit conversion to type if all the b values do, analogous to the behavior of tuples. {.a: b: i32 = 0} still has repeated punctuation, but seems much more readable than the current syntax. Also removes the possibility of getting the : vs = syntax wrong.
• {.a ~ b} with infix ~ meaning "matches". We would also consider allowing things like expr ~ pattern as a bool-valued expression to perform pattern matching. We want prefix ~ to mean destructive move, which this doesn't conflict with, but it's not ideal to use the same symbol for two unrelated things.
• {.a keyword b}, with keyword being is, match, matches, or like. A keyword may be too much ceremony here, especially in a large pattern with many field matches. is might be expected to be analogous to as, and isn't.
• {.a |> b}: feasible, although a little awkward to type on US keyboard layouts.

Any other information that you want to share?

See #7530 for more information, including discussion of several options.