PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

For v9+ (ESM) dev builds, the webpack config called useSourceMapFiles(), which overwrote env.sourceMap with 'inline-source-map'. This had two problems:

1. env.sourceMap was silently ignored. Since NativeScript 9, passing --env.sourceMap=<type> (e.g. source-map) on a dev build had no effect — the user's value was clobbered before getSourceMapType() ever saw it, so builds always emitted inline source maps regardless of what was requested.
2. File (external) source maps were effectively lost. Everything was forced inline, even though the Android and iOS runtimes are capable of loading .map files.

What is the new behavior?

The clobbering of env.sourceMap is removed. Instead, the v9+ dev-build path only adjusts a local defaultSourceMap fallback (now 'source-map'), which is used solely when the user hasn't specified env.sourceMap:

• env.sourceMap is always respected again. A user-provided --env.sourceMap=<type> (or =true/=false) now flows through getSourceMapType() unchanged, on both dev and production builds.
• File source maps are restored as the default for v9+ dev builds, so the Android/iOS runtimes once again handle external .map files properly instead of everything being inlined.