Support `alias this = _base;`

[ryuukk]

DScanner says this code is an error:

alias this = _base;

This is an allowed syntax since dlang version: 2.105.0 https://dlang.org/changelog/2.105.0.html#dmd.alias-this-syntax (1 august 2023)

While i type this i realize it probably should be reported in libdparse repo