Add text interpolation classes

As the first step of the text interpolation implementation, this commit
adds TextInterpolator and TextTemplate classes. These classes are not
yet used - that will come in the next MR. Once text interpolation is
fully implemented and has been tested under a feature flag, the original
Interpolation and Template classes will be removed and these new classes
will be renamed.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/433002