[parser] Extract for-over cursor declarations into clause_for_cursor

Extract multiple ways how to declare cursor variable(s) of for-over
loop into dedicated non-terminal allowing to merge `for-over` branches.

Merged for-over branches of:
- `for $scalar (...)`
- `for my $scalar (...)`
- `for my ($foo, $bar) (...)`
- `for \ $scalar (...)`
- `for my \ $scalar (..)`

Branch `for ()` must still remain as far as LALR(1) grammar will run
into shift/reduce conflict with `for (;;)` - there will be two possible
paths between `KW_FOR` and `(`:
- `KW_FOR remember '('` - from `for (;;)` branch
- `KW_FOR remember clause_for_over_cursor '(` - from `for ()` (using default cursor)

Author: Branislav Zahradník