More correct handling of strings/comments in Vim

Both of those start with a double quote (").

- if it's at BOL, it's a line comment,
- if it's in the middle of a line it's:
  - a string, if it's terminated by a double quote
  - a comment, if it's terminated by EOL

Strings can also contain escaped double quotes and line breaks
*followed* by a back slash (\).

Currently we don't respect the BOL condition, we only look if the
lexeme is ended by a quote. It means that a comment started at BOL
that contains a double quote will be mis-highlighted. Handling it
properly is a fragile pain in the ass with the current parser. We
lose BOL context when test for individual lexems in testRe(), so it
requires splitting the whole body of the language by lines and
treating them either as comments or as everything else.

Author: isagalaev

src/languages/vim.js

@@ -0,0 +1,4 @@
+<span class="hljs-comment">" comment</span>
+<span class="hljs-keyword">let</span> one = <span class="hljs-string">"string"</span> <span class="hljs-comment">" comment</span>
+<span class="hljs-keyword">let</span> two = <span class="hljs-string">"crazy
+\ string with a \" quote"</span>

test/markup/vim/strings-comments.expect.txt

@@ -0,0 +1,4 @@
+" comment
+let one = "string" " comment
+let two = "crazy
+\ string with a \" quote"