A JavaScript string value. JS strings are sequences of UTF-16 code units
with no validity requirement, so a value can contain unpaired surrogate
halves that Rust’s String cannot represent. JsString keeps the common
valid case as UTF-8 and falls back to code units only when the value is
ill-formed, so the compiler computes on true program values instead of
replacement characters or escape hatches.
Source location (matches Babel’s SourceLocation format)
This is the HIR source location, separate from AST’s BaseNode location.
GeneratedSource is represented as None.