Add completion for heredoc/xheredoc endings

[turboladen]

The actual text of the heredoc opening (ex. DOC in <<DOC\nDOC) doesn't get included in the AST. Two possible solutions:

• Pass the source into the completer to calculate the offset and extract it from the source.
• Use the ParseResult tokens, match on range to find the right token, and get it from there. I'd like to turn token parsing off though, so maybe not this.

Originally posted by @turboladen in #11 (comment)