fix: handle indented code fences in markdown transformation

[fresh3nough]

Fixes #1389

Problem

Running zx --verbose on a markdown file containing code fences indented inside list items (e.g. with 2 spaces) causes a TypeError: $(...) is not a function.

The transformMarkdown() parser checks for tab-indentation (tabRe) before checking for code fences (codeBlockRe). When a fenced code block is indented with 2+ spaces (common inside list items), the tab check matches first and the line is emitted as raw code rather than being recognized as a bash/js fence. The raw backticks in the output produce invalid JavaScript.

How to reproduce

Create a markdown file with a code fence inside a list item:

- item 1

  ```bash
  echo "hello"

Run: `zx --verbose file.md`

Result: `TypeError: $(...) is not a function`

## Fix

Per the CommonMark spec, code fences may be indented by 0-3 spaces. This change:

1. Reorders the `root` state to check for code fences (after stripping 0-3 leading spaces) **before** checking for tab indentation
2. Updates closing fence detection in `js`, `bash`, and `other` states to also strip 0-3 leading spaces before comparison

Tab-indented lines (4+ spaces or a tab character) continue to be handled as before.

## Test

Added a test case reproducing the exact markdown from issue #1389, verifying all three bash code blocks (including the indented one) are correctly converted to `await $\`` invocations.

node --experimental-transform-types --test test/md.test.ts

md

transformMarkdown()

transformMarkdown() handles indented code fences in list items (#1389)

tests 2, pass 2, fail 0

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- [x] **Tests**: I've run `test` and confirmed all tests succeed. Added tests to cover my changes if needed.
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[antongolub]

There are a lot of pitfalls here. I'll take care of it myself.

CC @Illuminati9

[antongolub]

Superseded by #1419