[dart] Split mustache file by serialization

[agilob]

No functional changes only code cleanup in mustache files so each serialization has own files, removes conditions from .mustache for readability.

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@kuhnroyal

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[agilob]

The whitespace change (removal before fromJson()) is only in json_serializable code which hasnt been released yet.

[kuhnroyal]

I started working on a new dio generator that would also support multiple serialization libraries here: #8869
We should use the same file structure for the partials, I have a separat sub folder serialization/built_value with the specific mustache files.

[agilob]

ok, will change it later today

[kuhnroyal]

Looks good, probably don't need the native/json_serializable prefix anymore for the partials.

[agilob]

i thought so too, but it failed to find inlined enums without it

[agilob]

@wing328 can we get this in pls?

[kuhnroyal]

Asked in Slack for a merge