Disallow multiple expressions in a placeable

[stasm]

Goal

Simplify the grammar and the AST.

Description

Placeables currently allow more than one expression separated by a comma. The resulting list must be implicitly formatted with a language-specific List formatter before being interpolated into the parent pattern. Separating with a comma conflicts with the planned changes to allowing lists as selectors to select-expression.

The implicit list is also a rare construct and at the same time it impacts the shape of the AST for every placeables. I suggest to remove this feature. The { LIST(…) } syntax can be used instead. In the future we can also consider a syntax for list literals but the placeable would still have only one expression.

Discussion

https://groups.google.com/forum/#!topic/mozilla.tools.l10n/YBNCSp7J0zU