Add joinEnv and joinUrl string functions and 2 new system vars

[solvingj]

Add two new system variables which map directly to platform specific go constants:

• PATH_LIST_SEPARATOR -> os.PathListSeparator
• FILE_PATH_SEPARATOR -> os.PathSeparator

Add joinEnv string function which is sibling to joinPath:

• strings.Join(elem, string(os.PathListSeparator))

Add joinUrl string function which is sibling to joinPath:

• path.Join(elem...)

Add all to docs

closes #2406

[trulede]

@solvingj there are quite a few functions already in https://go-task.github.io/slim-sprig/ and perhaps its better to use those existing list functions (join, append, prepend) since that gives a better control over how such lists/strings are generated.

The PathSeparator and PathListSeparator are useful however I would suggest following the existing most-predominate naming convention:

• PathSeparator -> osPathSeparator (note also the absence of "file")
• PathListSeparator -> osPathListSeparator

since the allcaps just looks ODD.

Observation: this could also be done with a .env file, introducing these constants (and others). Perhaps this PR is more effort than value.

[solvingj]

Up to @pd93 and @andreynering I suppose.