mldoc's astExportMarkdown loses backslash

Nobody has claimed this yet.

Following up to #112, when I use Mldoc's astExportMarkdown from js (via nbb), I observe that backslashes are removed from the resulting export. Steps to reproduce:

```sh
# Build latest mldoc
$ cd /your/local/mldoc
$ make
$ cp _build/default/js/lib.bc.js js/package/index.js

$ git clone https://github.com/cldwalker/nbb-clis && cd nbb-clis
# Install local mldoc until a new one is published
$ cd clis/mldoc
$ npm i /your/local/mldoc/js/package
$ cd -

# Roundtrip example failure
$ echo "- example postgres cmd: \dt" > basic.md
$ logseq-roundtrip basic.md basic2.md
$ diff bas*
1c1
< - example postgres cmd: \dt
---
> - example postgres cmd: dt
```
You can see that the resulting export removes the backslash. However, it seems backslashes aren't removed if there is whitespace after the backslash.
For example, `- alias apm='/Applications/Atom\ Beta.app/Contents/Resources/app/apm/bin/apm'` exports the same

Run the listed mldoc build and nbb roundtrip reproduction, then inspect the astExportMarkdown entry point and the generated js/package/index.js boundary. Done when a backslash in input such as `\dt` is preserved in the Markdown export and the reproduction diff no longer shows it removed.

Written by the indexing model from the issue text.