fix(cloudflare): keep default exports intact when auto-instrumenting

[isaacs]

The Vite transform rewrote export default <expr> to const __SENTRY_DEFAULT_EXPORT__ = <expr> and set the wrapped default export at the end of the module. That broke the module in four ways:

• A parenthesized default export, such as export default (handler);, became a syntax error. The rewrite replaced everything up to the expression, which dropped the ( and kept the ).
• An anonymous class, function or arrow took the name __SENTRY_DEFAULT_EXPORT__ in place of default. Libraries can read .name, as fix(cloudflare): Keep the user's class name when auto-wrapping an export #24700 found for named classes.
• export default class Handler {} lost the Handler binding, so other code in the module that used Handler threw.
• A circular import that read the default export after the original export default statement, but before the end of the module, threw.

The transform now replaces only the export default keywords, keeps a named class or function declaration as it is, and gives an anonymous one the name default by way of an object property.

The default export is set in two steps, as for export { handler as default }. Where the original statement set it, it is bound to the value as-is, so a circular import gets the handler. At the end of the module, it is rebound to the wrapper, so methods added to the handler after export default are instrumented too.

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