repl: support inherit statements and multiple bindings

[roberth]

Motivation

• Fixes allow inherit statements in nix repl #15053

This adds support for inherit statements and multiple bindings per
line in nix repl:

nix-repl> a = { x = 1; y = 2; }
nix-repl> inherit (a) x y
nix-repl> x + y
3

nix-repl> p = 1; q = 2;
nix-repl> p + q
3

Details

Design

The REPL now uses a parser-based approach instead of ad-hoc regex
matching. It tries parsing input in order:

1. As an expression (using the regular parser entry point)
2. As bindings (using a new REPL_BINDINGS entry point)
3. As bindings with an appended semicolon (for convenience)

The third attempt allows users to omit the trailing semicolon for
single statements like inherit (a) x or foo = 1.

A separate REPL_BINDINGS parser entry point is required because
combining expressions and bindings in the same production causes
reduce/reduce conflicts: ID . is ambiguous between attribute access
(a.b) and the start of a nested binding path (a.b = 1).

Alternatives considered

GLR parsing: Would resolve the ambiguity but risks parsing
performance regression for all Nix code, not just the REPL.

Duplicated grammar: A REPL-specific repl_binds production that
duplicates binds1 rules. Rejected to avoid maintenance burden and
grammar drift.

Single binding per line: Simpler but less useful. The current
approach reuses binds1 directly, getting multiple bindings and
nested attribute paths for free.

Tradeoffs

Binding input may be parsed twice (once as expression, then as
bindings). This is acceptable for interactive REPL use where input is
typically small and latency from parsing is negligible.

Error messages preserve the original input even when the semicolon-
appended parse is attempted, so users see inherit (a) y in errors
rather than inherit (a) y;.

Context

• Fixes allow inherit statements in nix repl #15053

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