Add config option to specify max file size

[brendan-kellam]

This PR adds the settings.maxFileSize option to the config schema, which allows users to specify the maximum file size (in bytes) that zoekt should index. E.g.,

{
    "$schema": "./schemas/v2/index.json",
    "settings": {
        "maxFileSize": 4194304 // Set size to 4MB (default 2MB)
    }
    // ...
}

All repositories in the config will use this file limit when indexing.