wasm: implement NO_LEADING_SPACES edge flag for lex children

[pdubroy]

• Addresses the corner case described in Accessing skipped whitespace #520 - we need metadata to record "this child in this context has no leading spaces"
• Same general approach as wasm: implement "edge metadata" to mark where leadingSpaces can appear #607, but slightly different implementation

[pkg-pr-new]

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commit: 3169003