feat: add Locale and LocaleSymbols for configurable date/time formatting

[algebraic-dev]

This PR adds Locale and LocaleSymbols for configurable date/time formatting. It also modifies alignedWeekOfMonth and weekOfYear so it contains a parameter to the first of the week.

This PR is useful for the TR35 formatting PR.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 0a6c31520b2ebaa6e71227454b46ebfb4986ae7b --onto 3c6317b6d77a565b4217532d1190ac6955dba842. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-29 06:42:47)
• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2026-04-29 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-29 23:51:37)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 0a6c31520b2ebaa6e71227454b46ebfb4986ae7b --onto 3fc99eef102549c743c6c63547983319bfae6f01. You can force reference manual CI using the force-manual-ci label. (2026-04-29 06:42:49)
• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2026-04-29 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-04-29 23:51:38)