Make SUBSTR(col, 1, n) = const_str sargable

Conditions looking like

SUBSTR(str_col, 1, n) = const_str 

where the prefix of str_col is compared against a constant value can be rewritten as range conditions as

const_str_low <= str_col <= const_str_high

which will improve the optimizer capabilities of building better execution plan (for example, using indexes on column str_col and applying range optimizations).
The similar transformation is already implemented for conditions like

str_col LIKE 'foo%'

aiming for the same goal.

Similar rewrite is implemented for DATE() and YEAR() function at MDEV-8320, and that approach may be used as a reference.