[!120] Allow escaped curly brackets in values

Merge branch 'feature/escapable-curly-brackets' into 'master'

ref:quality/dune-testtools Preventing key-dependent resolution. The use case I am implementing this for is quite expressive:

( ${CMAKE_CXX_COMPILER_ID} STREQUAL Clang ) AND ( {foo} STREQUAL bar ) | cmake_guard

Without this commit, meta ini would try to find replace CMAKE_CXX_COMPILER_ID.

See merge request quality/dune-testtools!120

(cherry picked from commit 65b82305)

c8a3e97f Allow escaped curly brackets in values