Added TypeScript supporting declaration file

[pouyakary]

Hi!
Thanks for the awesome tool. I wanted to use it in TypeScript so I wrote the declaration file for it and I tested the AST for all Markdown elements I knew. If you felt like something is missing tell me to fix it. Hope it helps

Have an awesome day :D
Pouya

[azu]

@pmkary Thanks for PR!
markdown-to-ast is a wrapper (and has a bit chaged for textlint) of remark.
It is beter to submit this PR to remark-parse I think.

FYI: syntax-tree/mdast: Markdown Abstract Syntax Tree format

[pouyakary]

@azu ah! I didn't know that. I just installed it because it was the first thing I found on the web that was good enough and I had to write the declaration as it was needed for me and felt like sending it to you didn't really noticed the dependencies :D I'll be checking that. Thanks for mentioning ;D