In Packet drawings, allow bit count as well as start-end

[BryanCrotazGivEnergy]

Proposal

Mermaid packet format insists on bit start -> bit end. This makes it very difficult to change your mind while generating a packet diagram, eg increasing one field from 16-32 bit.

As well as supporting
<s>-<e>: "name"
where s is the start bit and e is the end bit

Also support
<s>-<b>bits: "name"
<s>-1bit: "name"
where s is the start bit, b is a bit count

Short form is the most valuable
<b>bits: "name" follows on automatically from the previous bit. If this is the first field, it's assumed to start at bit 0.

Example

```mermaid
8bits: "Message Id"
16bits: "From Id"
32bits: "Command Data"
1bit: "write flag"
7bits: "reserved"
64-8bits: "new data"
```

Screenshots

No response

[zanedp]

I was considering using the "packet" type as a way to draw a memory map, which would be byte-scale instead of bit-scale. I wonder if a different syntax that didn't "tie" the diagram to a particular unit of measure would be more flexible? Shorting to b would make it so both "bits" and "bytes" make sense. But maybe doing something like +5 to mean it's relative to the previous thing would work and be intuitive?

+16: Header
+32: Data
+4: Footer

Using + is similar to "relative time" in PlantUML's timing diagram.