Source-level preprocessor for Snowball .sbl files.

Handles the two Snowball directives that must be resolved before tokenisation:

• stringescapes LB RB — declares that LB name RB inside string literals is a substitution reference (e.g. stringescapes {} makes {name} the substitution syntax).

• stringdef name 'value' — defines a named string alias, where the value may use {U+XXXX} Unicode escapes.

After preprocessing the returned source:

• All stringescapes and stringdef declarations are removed.

• Every string literal in the remaining source has its {U+XXXX} and {name} sequences expanded to real UTF-8 characters.

The resulting source can be fed directly to Snowball.Lexer.tokenize/1.