fix: align std.lines with official semantics

[He-Pin]

Motivation:
Official Jsonnet documents std.lines(arr) as concatenating an array of strings into a text file with a newline after each string. It is not a line-splitting function for string input.

The native sjsonnet implementation should therefore preserve array-to-text semantics and reject non-array input. This also resolves the behavioral confusion discussed in google/go-jsonnet#885 without treating the upstream issue thread as the specification.

Modification:

• Rewrite the native std.lines path to iterate over Val.Arr directly with a StringBuilder.
• Preserve existing std.join-compatible handling for null array elements.
• Add regression coverage for array input, null elements, and string-input rejection.

Result:
std.lines(["a", "b"]) follows the documented text-file construction behavior, including the trailing newline. std.lines("a\nb\n") remains a type error because the documented parameter is an array. The table below includes the old sjsonnet behavior from master before this PR.

Case	Official meaning	C++ jsonnet / go-jsonnet	jrsonnet	sjsonnet before this PR	sjsonnet after this PR
std.lines(["a", "b"])	array-to-text with trailing newlines	"a\nb\n"	"a\nb\n"	"a\nb\n"	"a\nb\n"
std.lines(["a", null, "b"])	preserve existing null-skipping join behavior	"a\nb\n"	"a\nb\n"	"a\nb\n"	"a\nb\n"
std.lines("a\nb\n")	invalid input type	type error	type error	[std.lines] Wrong parameter type: expected Array, got string	type error

The observed external behavior on master already matches the documented cases above; this PR keeps that behavior explicit with focused native-path tests and implementation cleanup.

References:

• Official Jsonnet std.lines documentation: https://jsonnet.org/ref/stdlib.html#std-lines
• go-jsonnet issue motivating the comparison: std.lines() fails on any string input: 'Unexpected type string, expected array' google/go-jsonnet#885