optional string-filler for DEFS

[ped7g]

idea from z88dk pull request: z88dk/z88dk#1834 (comment)

entries_array:
    DEFS 10, "name_1st"
> 6e 61 6d 65 5f 31 73 74 00 00
    DEFS 10, "name_2"
> 6e 61 6d 65 5f 32 00 00 00 00

• check if syntax is possible and not clashing with something else about DEFS or anything
• implement
• ???
• profit
• check how does this relates to structures, maybe this could have been done instead of the TEXT fields, or maybe it's still different (DS field doesn't have explicit initializer value in {} notation? Even then it may be useful to extend structure.DS too)

[ped7g]

syntax can follow structure TEXT (last explicit byte is also a filler) without requiring curly-braces.

Struct-DS seems low benefit as there is already TEXT field, so skip that one (and would have to record the whole text as init value and that's size limited, so it further complicates implementation).