pdl,bluetooth_packetgen: Change the syntax of size modifiers

Change the unit of size modifiers from bits to octets, and
restrict the syntax of to \+ *[0-9]+
This limits the use of size modifier to add declared packet fields
to a sized array or payload:

  _size_(value): 8,
  tag: 8,
  value: 8[+1],

In this example the tag is also counted in the value length.

Test: atest bluetooth_test_gd_unit packets_test_rust
Change-Id: Ib475f8259e10ae358cfdf8074f4012cfa7f95860