Use Base.require workaround in IJuliaExt (#4749)

* Always use Requires for IJulia

Addresses issue #4719.

* Move non-IJulia-dependent code out of IJuliaExt.jl

Reduces code loaded by @require while handling
IJulia via Requires only.

* Handle IJulia via extension if possible, but use Base.require

* Formatting update ext/IJuliaExt.jl

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

ext/IJuliaExt.jl

@@ -3,7 +3,8 @@ module IJuliaExt
 import Plots: @ext_imp_use, Plots, Plot
 using Base64
 
-@ext_imp_use :import IJulia
+const IJulia =
+    Base.require(Base.PkgId(Base.UUID("7073ff75-c697-5162-941a-fcdaad2a7d2a"), "IJulia"))
 
 function _init_ijulia_plotting()
     # IJulia is more stable with local file