Markdown: respect useTabs for indented code blocks and footnote bodies

[WhoamiI00]

Description

Fixes a subset of #8782 — when useTabs is enabled, Prettier was still emitting four literal spaces for indented code blocks and for the 4-column alignment used inside footnote definitions.

CommonMark explicitly treats a tab as advancing to the next 4-column stop (spec §2.2), so a single \t is structurally equivalent to four spaces for both of these cases — the document still parses as an indented code block / footnote continuation, and tabWidth doesn't change that (the 4-column rule is fixed by the spec, not by Prettier's setting).

List-item alignment is intentionally not changed in this PR: list prefixes (- , 1. , …) are not multiples of 4 columns, so substituting a tab there would actually break the structure. That part of #8782 needs a separate, more involved fix.

Before / after

<!-- Input (with `--use-tabs`) -->
# Indented code block

	first line
	second line

[^x]: Footnote

	Continuation paragraph.

<!-- Prettier stable (4 spaces — ignores --use-tabs) -->
# Indented code block

    first line
    second line

[^x]: Footnote

    Continuation paragraph.

<!-- Prettier main (tab — respects --use-tabs) -->
# Indented code block

	first line
	second line

[^x]: Footnote

	Continuation paragraph.

Checklist

• I've added tests to confirm my change works.
• (If changing the API or CLI) I've documented the changes I've made (in the docs/ directory).
• (If the change is user-facing) I've added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• I've read the contributing guidelines.
• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

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