remove ZDM from parser

Currently the parser uses ZDM internally, which got deprecated decades ago. While this is not a big deal for currently implemented sequence functions, it limits the expressiveness of newer or yet to come features, as a numercial 0 value is not possible.

ZDM should be removed from the parser logic and be replaced with appropriate default value handling on individual functions.

[jerch]

@Tyriar I think this needs to be scheduled for v5, as it would need integrators hooking own sequences into xterm.js to be aware of the -1 case to still handle the default case correctly.

I would create a PR for it, that would have to wait for v5 then (similar to the sound PR draft, which is also a v5 thingy).

[jerch]

Well ZDM defaults empty params to 0, which has a certain meaning in the context of the called function (most functions map it to 1). This basically makes 0 as numerical value unusable, unless its default meaning is zero as well.

ZDM itself is a relic from very early terminals in the 70s with very limited parsing power, and got removed from the specs in the 80s. We kinda inherited that mode in the parser, as all functions we currently have implemented fit the ZDM model (and it makes the parser slightly simpler). But there are newer functions, that dont follow that scheme anymore, beside the discourage of the specs.

Removing ZDM does change a few things, but would not affect any of the existing functions. Involved changes:

• map empty params to some other value in the parser than 0 (prolly -1, as it cannot be reached by normal means in a sequence)
• fix all functions with ZDM behavior, so -1|0 again maps to the default (e.g. -1|0 => 1 for cursor sequences)

With these changes 0 now can mean zero, while the sequence has some from zero different default value. Its fixing that “definition hole” of ZDM.