Support extension modifier on children declarations

[mbouaziz]

Summary

• Implements Feature request: extension children #1017: adds extension modifier support on children declarations
• Per-child syntax: | extension A marks individual children as extension classes
• Blanket syntax: extension children = | A | B marks all children as extension classes

Changes

• AST: Added chi_extension: ?FileRange to Child
• Parse tree: Added extension field to ChildClassTree, modifiers field to ChildrenTree
• Parser: Recognizes extension as a class member modifier, allows it on children and individual child classes
• Conversion: Propagates per-child and blanket extension to AST
• Expand: Passes chi_extension through to desugared Class_def
• Printer/skfmt: Prints extension on children and child classes

Test plan

• New test extension_children_per_child — validates | extension A() syntax
• New test extension_children_blanket — validates extension children = syntax
• Updated modifier_children error test — error message now says "Only 'extension' modifier is permitted"
• All 1702 existing tests pass

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