[KEYWORD] Add logical operators: and / or / not

[alphacrack]

There are no mappings for the logical operators and, or, not — conditions must be written with nested यदि blocks.

Suggested starting points (native speakers, please improve!):

• आ → and
• वा → or
• not is the tricky one: the obvious word नहि is already part of नहि त (else) and नहि त यदि (elif) — a bare नहि mapping must be ordered after those longer phrases in DEVNAGIRI_KEYWORD_MAP, and needs regression tests proving नहि त: still becomes else: (patterns apply in dict insertion order).

Steps:

1. Mappings in mappings.py with careful ordering
2. Tests: each operator alone, combined conditions, and the नहि / नहि त / नहि त यदि collision matrix
3. README keyword table + an example using a compound condition

Good first issue with one genuinely interesting edge case. Propose word choices in a comment first.