Fix PCRE-incompatible lookbehind in normal_statement

Unwrap the non-capturing group inside the lookbehind in
normal_statement.begin to allow PCRE to auto-split the
variable-length alternatives.

Before: (?<=(?:^|;|\||&|!|\(|\{|`))
After:  (?<=^|;|\||&|!|\(|\{|`)

This is the only regex in the grammar that PCRE cannot handle
natively. All other lookbehinds with mixed-width alternatives
are auto-split by PCRE at the top level.

Author: Chocapikk