Support arbitrary expressions

Currently, you can only interpolate a single variable name, not literals (e.g. `[url|/foo/#{1}|]`), constructors (e.g. `[url|/foo/#{True}|]`), or expressions (e.g. `[url|/foo/#{getThing asdf}|]`). I can't imagine common use-cases for these other expressions, but it might be useful for a minority of users.

Would require adding `haskell-src-meta` as a dependency, which should be hidden behind a cabal flag, since this is probably not a common use-case.

Start by reviewing how interpolation currently accepts a single variable name and how the Cabal configuration defines dependencies and flags. Assess adding haskell-src-meta behind an optional Cabal flag, then verify that literals, constructors, and expressions such as the examples in the issue are accepted while existing interpolation remains supported.

Written by the indexing model from the issue text.