replace_import doesn't work if there are no other AST nodes

[morgante]

If a file is only a single AST node, using replace_import fails.

Sample file:

import { ThemeProvider } from '@mui/styles';

Pattern:

engine marzano(0.1)
language js

`ThemeProvider` as $target where {
    $target <: replace_import($old, new=`"@mui/material/styles"`), 
}

Expected output:

import { ThemeProvider } from "@mui/material/styles";

Actual output:

import { ThemeProvider } from "@mui/material/styles";

import { ThemeProvider } from '@mui/styles';

Adding even a blank after the import is sufficient to restore the expected/correct behavior.

[morgante]

We likely need to use += or similar.