`text/unstable-dedent` still gives poor results when interpolating multi-line text into tagged template in certain edge cases

[lionel-rowe]

Describe the bug

text/unstable-dedent still gives poor results when interpolating multi-line text into tagged template in certain edge cases.

See also #6665 — the example from that specific issue was fixed, but behavior is still buggy in some edge cases. As with that issue, npm:string-dedent gives the expected result.

Steps to Reproduce

import { assertEquals } from 'jsr:@std/assert';
import { dedent as stdDedent } from 'jsr:@std/text@1.0.16/unstable-dedent';
import npmDedent from 'npm:string-dedent@3.0.2';

for (const [name, dedent] of Object.entries({
  'jsr:@std': stdDedent,
  'npm:string-dedent': npmDedent,
})) {
  Deno.test(name, async (t) => {
    const inner = dedent`
          [
            ...
      ...
          ]
    `;

    // ok
    await t.step('inner', () => assertEquals(inner, '    [\n      ...\n...\n    ]'));

    const outerV1 = dedent`
      a
          b
      ${inner}
    `;

    const outerV2 = dedent`
        a
            b
        ${inner}
    `;

    // throws for @std/dedent, ok for npm:string-dedent
    await t.step('outerV1', () => assertEquals(outerV1, `a\n    b\n${inner}`));

    // ok
    await t.step('outerV2', () => assertEquals(outerV2, `a\n    b\n${inner}`));
  });
}

Not sure why outerV1 and outerV2 give different results, as they should be exactly equivalent, the only difference being that the indent to be stripped is two spaces wider in V2.

Environment

text@1.0.16

[lionel-rowe]

Also, trailing newlines shouldn't be stripped from interpolated strings. Probably interpolated strings should always just be included verbatim (see also #6831)