feat: new Path library for Std

[algebraic-dev]

This PR implements `Std.Path' RFC

[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 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 84b251f7390c20a0a00a221050ab4a6e6c46a191. You can force Mathlib CI using the force-mathlib-ci label. (2026-06-30 02:52:04)

[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 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto bf755648d817af332909637a9ab0573dadef542f. You can force reference manual CI using the force-manual-ci label. (2026-06-30 02:52:05)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 28b99ec01f6275235fedecbac8e382cac682bf80. You can force reference manual CI using the force-manual-ci label. (2026-06-30 12:44:03)