feat: add text-matching assertions (expect_matches, expect_output_contains, expect_pcre_matches); bump to 0.5.0

[helly25]

Why

bashtest had no matcher for "does this (multi-line) captured text match a substring/regex" — the existing string matchers are exact-equality (expect_eq/expect_ne) and array-membership (expect_contains). That gap forces users of CLI-output tests to hand-roll printf '%s' "$out" | grep -qE ..., which is a footgun under set -o pipefail (which bashtest.sh sets and its examples mandate for test scripts): grep -q exits on the first match, the producer takes SIGPIPE, and pipefail flips a passing test into a flaky failure on large output.

What

Six new matchers, all following the existing matcher conventions (verbose-on-success, _BASHTEST_HAS_ERROR + actionable stderr on failure, 50-char display truncation):

Matcher	Engine	Notes
expect_output_contains / expect_output_not_contains	[[ == *x* ]]	literal substring; glob metacharacters are inert
expect_matches / expect_not_matches	bash [[ =~ ]] (ERE)	zero subprocess — structurally cannot SIGPIPE; ^/$ anchor the whole text, not per line (documented in the help text and doc comments)
expect_pcre_matches / expect_pcre_not_matches	external tool	detects grep -P → ggrep -P → pcre2grep → pcregrep once and caches; text is fed via a here-string (no pipe); fails with an install hint when no backend exists

Design notes:

• Two separately-named matchers per engine instead of an engine-selection flag: the name is the selector, expect_matches stays dependency-free/portable, and PCRE (\d, lookahead, non-greedy) is an explicit opt-in.
• The output_ prefix on the substring matchers avoids collision with the array-membership expect_contains.
• Also documents the SIGPIPE-under-pipefail pitfall in the README and extends bazel run //bashtest:bashtest_help.

Backwards-compatible API addition → minor bump to 0.5.0 (MODULE.bazel + CHANGELOG).

Testing

• New test:: functions in bashtest_test.sh mirroring the existing pattern (positive/negative cases, runner-counter integrity checks). PCRE cases gate on backend availability: positive-path assertions where a backend exists (Linux CI), graceful-failure assertion on stock macOS (BSD grep has no -P).
• Verified matcher semantics empirically on macOS bash 3.2 (oldest supported bash), including whole-text anchoring (^SECOND against $'a\nSECOND\nc' correctly does not match), literal-glob inertness, and the positive PCRE path via pcre2grep.
• bazel test //...: 3/3 pass. shellcheck 0.11.0 clean; all pre-commit hooks pass.