Now that Assembler supports multiple exporters, it doesn't make sense to always look for `antora-assembler.yml` by default. That means the exporter extension almost always has to be configured. Instead, Assembler should look for the profile-specific configuration file by default (if none is specified). For example, `antora-assembler-pdf.yml`. If this file is not found, Assembler could still fall back to looking for `antora-assembler.yml`.