Add a variable that resolves to the absolute path to the current file

New feature

Currently, all the paths in configuration files are relative to the nextflow script being executed: baseDir, projectDir and ./ all resolve to the same thing. I'm proposing adding a variable called something like thisDir that would resolve to the directory containing the file of the nextflow config file being read.

Use case

This would decouple config files from the script being ran to a greater extent. When I write a config file, I know where it is and where the paths it refers to are in relation to it. I imagine this is especially true when there are multiple config files being evaluated sequentially as described at the top of https://www.nextflow.io/docs/latest/config.html

My main use case is to allow me to have one nextflow.config in my project root defining the location of assets despite having multiple workflow scripts (I.E. a main workflow and other workflows wrapping subworkflows for testing purposes)

This would solve the issue described here: https://github.com/nextflow-io/nextflow/issues/1858

Start by tracing how Nextflow resolves baseDir, projectDir, and relative paths while reading configuration files. Define the behavior for a variable representing the directory of each config file, including sequentially evaluated configs, and verify that paths resolve relative to the correct config location.