Add Mexico support to the CLI

[leenyburger]

Stacked on top of #5 (UK support). Adds Mexico as a fourth accepted --country value.

Mexico is simpler than the UK addition: it's not access-gated (only GB is) and has no country-specific field appends, so this is purely the country flag + docs + tests.

Closes ENG-1113

Changes

• distance --country (normalizeCountry): accepts mexico (case-insensitive) → appends "Mexico". Both distance and distance-matrix warning strings and flag help updated.
• geocode --country usage text: now lists Mexico.
• README + bundled skill: Mexico added to the --country rows/hints.
• Tests: Mexico cases in TestAppendCountry.

Tests

gofmt clean · go build ./... · go test -race ./... all pass.

Note: review/merge #5 first — this PR is stacked on it.

[leenyburger]

This change is part of the following stack:

• Add Mexico support to the CLI #6 ◀

_{Change managed by git-spice.}

[leenyburger]

We changed the CLI so it doesn't do country client side validation, so this PR is no longer necessary.