[mlir][Pass] Handle spaces in pipeline strings

An user might want to add extra spaces for better readability, e.g:
```
mypm = pm.PassManager.parse(f"""builtin.module(
    mypass1,
        func.func(mypass2,mypass3)
)""")
```
GitHub issue #59151

The parser was not taking into account the possibility of spaces after
`)`or `}`

Differential Revision: https://reviews.llvm.org/D142821