docs: add error_matches example to README

[Copilot]

The top-level example block in the README was missing a demonstration of assert.error_matches, leaving it undocumented alongside the other quick-start assertions.

Changes

• README.md: Added one line to the existing top example block showing assert.error_matches with a function that throws an error and a pattern that matches part of the message:

assert.error_matches(function() error("foo bar") end, "foo")

Original prompt

Add a README example for the error_matches assertion in the repository lunarmodules/luassert.

Context:

• The user wants a pull request, not just an explanation.
• The relevant assertion implementation is in src/assertions.lua.
• error_matches(state, arguments, level) works by:
  • expecting at least two arguments: a callable and a string pattern;
  • executing the callable with pcall(func);
  • if the callable does not error, the assertion fails;
  • if the error value is a string, it strips any leading path/to/file:line: prefix;
  • it optionally accepts an error message plus init and plain arguments similar to string.match / string.find behavior;
  • if plain is truthy, it checks err_actual:find(pattern, init, plain) ~= nil;
  • otherwise it checks err_actual:match(pattern, init) ~= nil.

Task:

• Update the README to add a concise example near the top where the README currently lists a few examples.
• The example should demonstrate using assert.error_matches(...) with a function that throws an error and a pattern that matches part of the error message.
• Keep the style consistent with the existing README examples.
• Avoid broad documentation rewrites; make the smallest clear addition needed.
• Create a pull request with the change.

Repository: lunarmodules/luassert
Base branch: master

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: how does the error_matches assertion work? create a PR to add an example to the readme (at the top there is a list of a few examples)

Assistant: I’m waiting for your confirmation on the pull request creation step—once you approve it, I’ll continue.

This pull request was created from Copilot chat.