Update swc-defaults.ts to apply tsconfig source map options.

[ianzone]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A
if tsconfig has sourcemap enabled and i don't use swcrc, the sourcemap won't be generated.

What is the new behavior?

swc will apply tsconfig sourcemap setting.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

lgtm