fix: Add official escapeStringXml stdlib alias

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Summary

• add the official std.escapeStringXml spelling alongside the existing std.escapeStringXML alias
• share the XML escaping implementation between both names
• update playground completions and add compatibility coverage

Tests

• ./mill --no-server 'sjsonnet.jvm[3.3.7]'.test
• ./mill --no-server __.checkFormat

[He-Pin]

Closing after checking google/jsonnet's stdlib/std.jsonnet and test_suite/stdlib.jsonnet: the canonical function name is std.escapeStringXML. The lowercase std.escapeStringXml spelling is a documentation-generation mismatch upstream, so sjsonnet should not add this alias here.