The `sourcepath()` function determines the path of the currently executed
ucode script file, optionally only the directory portion.

By specifying the a depth parameter, the owning files of functions further
up the call stack can be determined.

Returns a string containing the path (or directory) of the running ucode
source file.

Returns `null` if the path is indeterminate.

-- Testcase --
{%
	let output = render("files/include/level1.uc");

	// replace dynamic testfiles path with placeholder for stable output
	output = replace(output, TESTFILES_PATH, "...");

	print(output);
%}
-- End --

-- File include/level1.uc --
This is the 1st level include.

{% include("level2.uc") %}
-- End --

-- File include/level2.uc --
This is the 2nd level include.

{% include("level3.uc") %}
-- End --

-- File include/level3.uc --
This is the 3rd level include.

{% for (let depth in [0, 1, 2, 3]): %}
Depth {{ depth }}:
  Path:      {{ sourcepath(depth, false) || "indeterminate" }}
  Directory: {{ sourcepath(depth, true)  || "indeterminate" }}

{% endfor %}
-- End --

-- Expect stdout --
This is the 1st level include.

This is the 2nd level include.

This is the 3rd level include.

Depth 0:
  Path:      .../include/level3.uc
  Directory: .../include

Depth 1:
  Path:      .../include/level2.uc
  Directory: .../include

Depth 2:
  Path:      .../include/level1.uc
  Directory: .../include

Depth 3:
  Path:      indeterminate
  Directory: indeterminate

-- End --