Fix undeclared variable error

[raydiak]

Previously it would show the cursor in a wrong (later) position and give inappropriate "expecting any of" suggestions based on that position. I'm a bit unclear on the details of this precursor thing, so please do review carefully before merging in case this is the wrong way to fix it, but what I saw seemed to suggest that precursor may be intended for these kinds of cases.

[AlexDaniel]

It feels like this pull request lacks the example:

$test = 1;

===SORRY!=== Error while compiling ./test.pl
Variable '$test' is not declared
at ./test.pl:9
------> $test⏏ = 1;
    expecting any of:
        postfix

[raydiak]

Thanks, good call...I had meant to at least include a link to the discussion at http://irclog.perlgeek.de/perl6/2015-05-08#i_10570744 but apparently forgot. The above example after this patch renders as:

===SORRY!=== Error while compiling -e
Variable '$test' is not declared
at -e:1
------> use strict; ⏏$test = 1;

[raydiak]

Figured it couldn't hurt to add one more example of misbehavior to show the confusion a little more fully:

> perl6 -e 'use strict; $foo.bar'
===SORRY!=== Error while compiling -e
Variable '$foo' is not declared
at -e:1
------> use strict; $foo.bar⏏<EOL>
    expecting any of:
        method arguments

[raydiak]

wrt $/.PRECURSOR, I found https://github.com/perl6/nqp/blob/5e9f7b8c563d3601961751aec03228325e33d76d/src/QRegex/Cursor.nqp#L731 which seems to fit the vague assumptions I was making