A lot of strings that are going to be passed to `getStringWidth()`
are ASCII strings, for which the calculation is rather easy and
calling into C++ can be skipped.

                                                       confidence improvement accuracy (*)    (**)   (***)
     misc/getstringwidth.js n=100000 type='ascii'            ***    328.99 %      ±21.73% ±29.25% ±38.77%
     misc/getstringwidth.js n=100000 type='emojiseq'                  2.94 %       ±7.66% ±10.19% ±13.26%
     misc/getstringwidth.js n=100000 type='fullwidth'                 4.70 %       ±5.64%  ±7.50%  ±9.76%