Allow parenthesized element expressions.

Currently list literal element expressions are either `if ...` or `for ...` or plain expressions. We do not allow parentheses around an `if ...` element, so the following:
```dart
var l = [ if (cond1) (if (cond2) 0) else if (cond3) 1 ];
```
is not allowed. That is inconvenient because we also do not have a "no element" element that is briefer than `...[]`, so the way to write the above logic today is:
```dart
var l = [ if (cond1) if (cond2) 0 else ...[] else if (cond3) 1 ];
```

We should consider allowing parentheses around all element expressions. The only place it actually matters is delimiting an `if` with no else, but that is an annoying surprise in the current grammar.

The change would be adding the following `elements` production:
```ebnf
::= `(' `)' | ...
```
which is ambiguous with parenthesized expressions, but since any ambiguous element/expression means the same thing whether it's an element or an expression, it doesn't matter how we resolve the ambiguity.

Optionally, also allow *multiple values*, comma separated, inside an "elements parenthesis":
```dart
var l = [if (cond1) (value1, value2, value3)];
```
which is currently not something we can write shorter than:
```dart
var l = [if (cond1) ...[value1, value2, value3]];
```
The spread-literal does work, so it's not strictly necessary, but could be convenient. Effectively we would instead change the grammar to:
```ebnf
::= `(' `)' | ...
```
(where the `` production allows trailing commas).
That grammar is no more ambiguous than the one above, any presence of a comma will definitely disambiguate towards the element production.

(Edit: Added multiple-values option and grammar).

Start with the and grammar productions described in the issue, comparing parenthesized if elements with ordinary expressions and spreads. Decide whether the change covers only single parenthesized elements or also comma-separated values; done means the grammar specifies the accepted examples and resolves their ambiguity.

Written by the indexing model from the issue text.