fix: add is_base_form to FrenchLemmatizer to prevent over-lemmatizing infinitives

[RudrenduPaul]

Description

Fixes #7320

French infinitives ending in -dre, -re etc. (e.g. descendre, prendre) were incorrectly lemmatized to descendrer, prendrer because suffix rules like e → er were applied to them after no match was found in the lookup tables.

Root cause: FrenchLemmatizer.rule_lemmatize fully overrides the parent class method and does not call is_base_form before applying suffix rules. The parent Lemmatizer.rule_lemmatize checks is_base_form before the rules loop (line 190), but the French override skips that.

Fix:

1. Added is_base_form() to FrenchLemmatizer — returns True when VerbForm=Inf in the token's morphology (same pattern as EnglishLemmatizer)
2. Added a call to self.is_base_form(token) at the top of FrenchLemmatizer.rule_lemmatize (after the space/eol early return), mirroring the parent class pattern

French infinitives are now returned as-is (lowercased) without suffix rule application.

Types of change

• Bug fix

Checklist

• I confirm that I have the right to submit this contribution under the project's open-source license
• I ran the tests, and all new and existing tests passed
• My changes don't require a change to the documentation, or if they do, I've added all required information