fix(text): preserve indent inside substituted values in dedent (#6830)

[LeSingh1]

Fixes #6830 (and the residual cases from #6665).

Problem

dedent computed the common indent using the template joined with a single-char \"x\" placeholder, then stripped that indent from the fully substituted input string. When a multi-line substitution's lines happened to start with whitespace matching the outer template's computed indent, those leading spaces were stripped too — silently breaking the substituted value.

Reproduction from the issue: inner dedents correctly to \" [\\n ...\\n...\\n ]\". Interpolating it back into an outer dedent with 6-space outer indent eats the 6 leading spaces from inner's \" ...\" line, rendering it as \"...\". V1 and V2 in the issue body diverge even though they should be identical.

Fix

Restructure the tagged-template branch to apply indent stripping per literal template part (the entries in input.raw), then interleave substitution values verbatim. The first literal part also gets a start-of-template strip for templates that begin with content (no leading newline). String-input mode is unchanged.

Concretely, instead of building the full substituted string and running replaceAll(^indent/gmu, \"\") on it, the new code runs replaceAll(\\n${indent}/gu, \"\\n\") (plus a once-only ^${indent} for the very first part) on each input.raw[i] and then concatenates the substitution value after each processed part.

Tests

Two new cases in text/unstable_dedent_test.ts:

• preserves indentation inside multi-line substitution values (#6830) — the exact V1/V2 reproducer from the issue. Both must equal \a\n b\n${inner}``.
• does not strip indent that originated inside a substituted value — narrower regression: ${\" leading-four-spaces\"} inside a 6-space outer template must keep its 4 leading spaces.

$ deno test --allow-read text/unstable_dedent_test.ts
ok | 13 passed (84 steps) | 0 failed (19ms)

$ deno fmt text/unstable_dedent.ts text/unstable_dedent_test.ts
Checked 2 files

$ deno lint text/unstable_dedent.ts text/unstable_dedent_test.ts && deno check text/unstable_dedent.ts text/unstable_dedent_test.ts
Checked 2 files
Check text/unstable_dedent.ts
Check text/unstable_dedent_test.ts