String interpolation within AQL

We need two operators for strings: deconstruction and construction. The latter is done like in many modern languages with string interpolation. The syntax is

`...{<expr>}...`

A literal { or backquote is inserted by interpolating a regular string: hello {'{'} world }.

Rationale

Since backquotes are needed to trigger interpolation, the quoting rules for normal strings can stay extremely simple. Quoting the contents of a string interpolation is also quite simple because it affects only two characters, which need to be replaced by a short string each.

Backslash quoting would add another character that needs to be quoted and that needs to be quoted within many source languages as well, leading to \\\\ cascades.

The interpolation syntax is deliberately chosen to minimise collision with the interpolation syntax of other languages.