v1.0: Remove Regex, keep only Regexp for clarity

[kolkov]

Suggestion from Ben Hoyt (GoAWK author).

Current state: We have both:

• Regex - original type name
• Regexp - alias for stdlib compatibility (v0.8.1)

Problem: Having both names causes confusion.

Proposal for v1.0:

• Remove type Regex
• Keep only type Regexp as the canonical type
• Update all code to use Regexp
• This matches stdlib naming exactly

Migration path:

• v0.8.1+: Both work (via alias)
• v0.9.x: Deprecate Regex, warn users
• v1.0: Remove Regex entirely

Related: benhoyt/goawk#264 (comment)