fix: keep manifestXmlJsonml output well-formed XML

[He-Pin]

Motivation:
Jsonnet documents std.manifestXmlJsonml(value) as converting a JsonML-encoded value to a string containing XML. JsonML elements are array-shaped values, and XML 1.0 start-tags, end-tags, empty-element tags, attributes, and character data impose stricter rules than the previous renderer enforced.

Before this PR, sjsonnet delegated rendering to scalatags. That rejected some valid XML names such as dotted element names, surfaced invalid names as generic internal errors, rejected array/object attribute values even though other implementations stringify them, accepted a bare top-level string even though it is not a JsonML element, and silently dropped XML-forbidden control characters.

Modification:

• Replace the scalatags-dependent manifestXmlJsonml rendering path with a small XML renderer for JsonML.
• Validate element and attribute names against the XML 1.0 Name production.
• Reject top-level non-array values with the same JsonML-shape error used by the official stdlib.
• Reject characters outside the XML 1.0 Char production instead of silently dropping them.
• Stringify array/object attribute values as compact JSON and escape them as XML attribute text.
• Preserve XML escaping for text and scalar attribute values, including supplementary Unicode characters.
• Add regression tests for empty/invalid names, valid XML names that scalatags rejected, array/object attribute values, top-level non-array input, invalid XML characters, and supplementary Unicode text.

Result:
sjsonnet now follows the documented "string containing the XML" contract even where other implementations currently emit strings that are not well-formed XML. The table below includes the old sjsonnet behavior from master before this PR.

Case	C++ jsonnet observed	go-jsonnet observed	jrsonnet observed	sjsonnet before this PR	sjsonnet after this PR
std.manifestXmlJsonml(["", {}])	"<></>" invalid XML	"<></>" invalid XML	"<></>" invalid XML	Internal Error from scalatags invalid tag name	invalid XML tag name ""
std.manifestXmlJsonml(["div", {data:{x:1}}])	"<div data=\"{\"x\": 1}\"></div>" invalid XML attribute quoting	same invalid XML	same invalid XML	unsupported attribute type object	"<div data=\"{&quot;x&quot;:1}\"></div>"
std.manifestXmlJsonml(["div", {}, "a&b"])	"<div>a&b</div>" invalid XML text escaping	same invalid XML	same invalid XML	"<div>a&amp;b</div>"	"<div>a&amp;b</div>"
std.manifestXmlJsonml(["child.node"])	"<child.node></child.node>"	same	same	Internal Error from scalatags invalid tag name	"<child.node></child.node>"
std.manifestXmlJsonml(["tag", {"bad attr": "x"}])	"<tag bad attr=\"x\"></tag>" invalid XML attribute name	same invalid XML	same invalid XML	Internal Error from scalatags invalid attribute name	invalid XML attribute name "bad attr"
std.manifestXmlJsonml("text")	Expected a JSONML value (an array), got string	same	same	"text"	Expected a JSONML value (an array), got string
std.manifestXmlJsonml(["tag", std.char(1)])	raw U+0001 in <tag> text, invalid XML character	same invalid XML	same invalid XML	"<tag></tag>" after silently dropping U+0001	invalid XML character U+0001
std.manifestXmlJsonml(["emoji", {value: std.char(128512)}, std.char(128512)])	"<emoji value=\"😀\">😀</emoji>"	same	same	same	same

This PR is intentionally not an "accept empty tags" compatibility change. Empty XML elements are valid only when they still have a valid XML element name, for example <tag></tag> or <tag/>.

References:

• Official Jsonnet std.manifestXmlJsonml documentation: https://jsonnet.org/ref/stdlib.html#std-manifestXmlJsonml
• XML 1.0 Name production: https://www.w3.org/TR/xml/#NT-Name
• XML 1.0 character range: https://www.w3.org/TR/xml/#charsets
• XML 1.0 start-tags, end-tags, and empty-element tags: https://www.w3.org/TR/xml/#sec-starttags
• JsonML syntax: https://www.jsonml.org/syntax/