select: Add new t:@<name> syntax

[povik]

What are the reasons/motivation for this change?

Sample use case: you identify buffer cells in a standard cell library with the following command (requires #4625)

cellmatch -lut_attrs *
select -set buffers a:lut=2'b10 %m

now you want to see the buffer cells instanced in a design. This new syntax allows you to do

stat t:@buffers

[povik]

To select instances with types starting with @ one can still do t:\@foo, I checked

[widlarizer]

I'm at my wit's end as to why this wouldn't work, so I pushed the reproducer directly as a test

select -list t:@buffers
# yosys prints top/buf
select -assert-count 0 top/not t:@buffers
# yosys prints:
# ERROR: Assertion failed: selection contains 2 elements instead of the asserted 0: top/not t:@buffers
# Selection contains:
# top/not
# top/buf

Edit: forgot -assert-count only takes the single count argument and not a type argument