Verify current working directory

[ProbablyManuel]

Is your feature request related to a problem? Please describe.

One of the most frequent Reqtificator support requests is from users who set the working directory to the folder where their mod manager unpacked Requiem instead of Skyrim's data folder.

Describe the solution you'd like

Add a file existence check for Skyrim.esm to verify that the Reqtificator runs in the correct folder. If the check fails, display an error message instructing the user to set the working directory to Skyrim's data folder, as detailed in the installation guide.

[ProbablyManuel]

Already implemented in #6