Problem:

`vim.validate()` takes two forms when it only needs one.

Solution:

- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
  of the `optional` argument.

- Also fix `vim.validate()` for PUC Lua when showing errors for values
  that aren't string or number.

Problem:
User cannot configure the tool used by `vim.ui.open` (or `gx`). With
netrw this was supported by `g:netrw_browsex_viewer`.

Solution:
Introduce `opts.cmd`. Users that want to set this globally can
monkey-patch `vim.ui.open` in the same way described at `:help vim.paste()`.

Fixes https://github.com/neovim/neovim/issues/29488

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

xdg-open is usually not installed in WSL. But if the user deliberately
installs it, presumably they want to prioritize it.

For context, see https://github.com/neovim/neovim/pull/24738. Before
that PR, Nvim did not correctly handle captures with quantifiers. That
PR made the correct behavior opt-in to minimize breaking changes, with
the intention that the correct behavior would eventually become the
default. Users can still opt-in to the old (incorrect) behavior for now,
but this option will eventually be removed completely.

BREAKING CHANGE: Any plugin which uses `Query:iter_matches()` must
update their call sites to expect an array of nodes in the `match`
table, rather than a single node.

Fixes: #30220

Use the "url" extmark attribute as well as the "url" tree-sitter
metadata key to determine if the cursor is over something Nvim considers
a URL.