[Feature request] `SIZEOF` and `STARTOF` for section types

[Rangi42]

For example, SIZEOF(SRAM) == $2000 and STARTOF(SRAM) == $a000.

These would have to be link-time values with new RPN opcodes, since sectionTypeInfo can be updated by some linker settings.

Alternatively, we could wait for hardware description files to be implemented, and require those to make the final section type metadata available at assembly time.