lint: add post-increment-decrement gas lint

[ParthSinghPS]

What this PR does

Adds a new post-increment-decrement lint rule in the gas category that flags
i++ and i-- when used in for loop update expressions, suggesting ++i and
--i instead.

Why

Post-increment (i++) and post-decrement (i--) return the value before
modification, requiring an extra copy operation under the hood. In for loop
update expressions the return value is always discarded, so the copy is
pure overhead. Pre-increment (++i) skips the copy entirely.

The lint is intentionally scoped to for loop update expressions only. Outside
of loops, i++ vs ++i is a semantic choice the developer may have made
deliberately.

Changes

• crates/lint/src/sol/gas/increment.rs: new EarlyLintPass implementation
• crates/lint/src/sol/gas/mod.rs: registered the new lint
• crates/lint/testdata/PostIncrementDecrement.sol: test cases covering
  triggering and non-triggering patterns
• crates/lint/testdata/PostIncrementDecrement.stderr: blessed test output