Add __filename/__dirname constants (like Node.js)

[mstoykov]

It will be useful if we can have consts such as nodejs __filename and __dirname in order to be easier to open() files relative to the current file instead of the current working directory.

[na--]

We've discussed this elsewhere, but we don't want to add new magic constants to our runtime. Constants like __filename and __dirname that overlap with the NodeJS runtime are even less desirable, since they might cause some poor feature detection JS logic to mistake k6 with Node...

Instead, we should probably tackle this by adding functions in some internal k6 module (e.g. k6/execution) that return the current working directory and the script file name. So, this falls under #1320, and I'll make a comment there about it after closing this issue.

[sarasensible]

Workaround to obtain the absolute path to a file when you know what the relative path would be from the root but you don't know what directory level you'll be accessing it from:

const dirs = ['path','to','file'];
const current_dirs = __ENV.PWD.split("/")
const root_dir = current_dirs.filter(x => !dirs.includes(x));

const locationsFile = `${root_dir.join("/")}/${dirs.join("/")}/thefile.txt`;