fix(mojo): grow string buffer

[P403n1x87]

We grow the string buffer if a string carried by the MOJO stream overfills it.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.71%. Comparing base (18e0d79) to head (dbc0580).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
austin/format/mojo.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
- Coverage   71.82%   71.71%   -0.11%     
==========================================
  Files          20       20              
  Lines        1817     1821       +4     
  Branches      225      225              
==========================================
+ Hits         1305     1306       +1     
- Misses        428      431       +3     
  Partials       84       84              

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