🐛 Fix prime combined with subscripts or superscripts

[roniemartinez]

Fixes #392

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (896229c) 100.00% compared to head (cf33161) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #395 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 6 6 Lines 929 936 +7 ========================================= + Hits 929 936 +7 ``` | [Impacted Files](https://app.codecov.io/gh/roniemartinez/latex2mathml/pull/395?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ronie+Martinez) | Coverage Δ | | |---|---|---| | [latex2mathml/walker.py](https://app.codecov.io/gh/roniemartinez/latex2mathml/pull/395?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ronie+Martinez#diff-bGF0ZXgybWF0aG1sL3dhbGtlci5weQ==) | `100.00% <100.00%> (ø)` | |

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