Alter regexp on initialize to avoid extra ast pass

[composerinteralia]

Along the same lines as #38901, #39903, and #39914, this commit removes
one pass through the journey ast in an attempt to improve application
boot time.

Before this commit, Mapper#path would iterate through the ast to alter
the regex for custom routes. With this commit we move the regex
alterations to initialize, where we were already iterating through the
ast.

The Benchmark for this change is similar to what we have been seeing in
the other PRs.

Warming up --------------------------------------
               after    13.121k i/100ms
              before     7.416k i/100ms
Calculating -------------------------------------
               after    128.469k (± 3.1%) i/s -    642.929k in 5.009391s
              before     76.561k (± 1.8%) i/s -    385.632k in 5.038677s

Comparison:
               after:   128469.4 i/s
              before:    76560.8 i/s - 1.68x  (± 0.00) slower

Calculating -------------------------------------
               after   160.000  memsize (     0.000  retained)
                         3.000  objects (     0.000  retained)
                         0.000  strings (     0.000  retained)
              before   360.000  memsize (     0.000  retained)
                         6.000  objects (     0.000  retained)
                         0.000  strings (     0.000  retained)

Comparison:
               after:        160 allocated
              before:        360 allocated - 2.25x more

[eugeneius]

Thanks Daniel! 🎉