Checks if the given string is an absolute URL.
]add IsURLjulia> using IsURL
julia> isurl("https://julialang.org")
true
julia> isurl("mailto:someone@example.com")
true
julia> isurl("/foo/bar")
false
julia> isrelativeurl("../path/to/directory")
true
julia> isrelativeurl("./__file__")
true
julia> isrelativeurl("foo:bar")
false