Have `sync` show as a keyword when used on a block

[jabraham17]

The Chapel language allows sync in two contexts. It can be used as a type modifier, but it can also be used on a block.

// type
var x: sync int;

// type
proc foo(x: sync int) { }

// block
sync begin { }

// block
sync { }

Currently the highlighter treats all sync keywords as the type version, which can look weird with some syntax highlight themes that distinguish between types and plain keywords.

Ideally, the highlighter would distinguish between the two. However doing this with only regex is non-trivial.