Add macro always_everywhere

[oschulz]

This enables

using Distributed, LegendDataManagement

@always_everywhere begin
    using Distributions

    get_global_value() = 42
end

dist = Normal()

legend_addprocs()

worker = last(workers())
remotecall_fetch(get_global_value, worker)
@fetchfrom worker Normal()

even though the last worker was added after @always_everywhere. This way, using and import statements and other init code, in scripts that use distributed computing, can just be somewhere at the top and don't have to be repeated after adding workers.

CC @theHenks

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 35.10%. Comparing base (fb3c66b) to head (ed7f6f2).

Files	Patch %	Lines
src/workers.jl	0.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   35.22%   35.10%   -0.12%     
==========================================
  Files          25       25              
  Lines        1482     1487       +5     
==========================================
  Hits          522      522              
- Misses        960      965       +5     

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