Previously it depended on QODEASSIST_QT_CREATOR_VERSION_* flags being
passed to cmake during build process. Making it standalone saves time
for the users.
chore: Add unit tests for DocumentContextReader
The tests are based on GTest like some tests in Qt Creator itself, which
makes it easy to run as full Qt Creator does not need to be started.
This will allow to add code conditional on the Qt Creator version to the
plugin codebase. The Qt Creator version will be passed from the build
script automatically. This will also allow to easily extend the Github
Actions job matrix to create releases for more than one Qt Creator
version.
Using QT_VERSION_CHECK allows to reuse existing Qt patterns of checking
versions.
Code has been tested by invoking QODEASSIST_QT_CREATOR_VERSION in code.
