fix: normalize config context loaded by jiti

[stormslowly]

Why

jiti rewrites __dirname to a POSIX-style path on Windows (D:/a/project), so a config written as context: __dirname reaches the bundler with forward slashes. rspack keeps that value as the resolver base and emits mixed-separator dependency paths, while watchpack rebuilds incoming OS event paths with path.join:

registered: d:/a\project\src\app.tsx   (mixed, from a forward-slash context)
fs event  : d:\a\project\src\app.tsx   (path.join => all backslash)
=> mismatch => the change is ignored

That is web-infra-dev/rspack#14446: on Windows, file watching stops after the first change.

What

Normalize the context field of the loaded config — every entry when the config is an array — to the native separator, and only when the config went through jiti. A natively loaded config already sees a native __dirname, so it is left untouched.

normalizeContext lives in src/jiti.ts so the whole workaround can be dropped in one place once jiti stops rewriting __dirname.

loader	context: 'D:/a/project' on Windows
jiti	D:\a\project
native	D:/a/project (unchanged)