Fix missing dot in import.defer/import.source codegen

[Copilot]

The codegen was incorrectly printing import phase expressions without the dot separator. Consider:

import.defer("foo");
import.source("bar");

Was being printed as:

importdefer("foo");
importsource("bar");

This fix adds the missing dot (.) between import and the phase string in the ImportExpression codegen implementation. The formatter was already handling this correctly, but the codegen was not consistent.

The change is minimal and surgical - adding a single line to print the dot before the phase string, ensuring the output matches the expected JavaScript syntax for import phase expressions.

Fixes #13971.

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[graphite-app]

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[Boshen]

Closed by #13975