t: extract chainlint's parser into shared module

Move chainlint.pl's Lexer, ShellParser, and ScriptParser into a
shared module (lib-shell-parser.pl) so other lint tools can reuse
the same shell parsing infrastructure.

ScriptParser's check_test() becomes a no-op in the shared module.
chainlint.pl defines ChainlintParser (extending ScriptParser)
with the &&-chain check_test() implementation.

No functional change: chainlint produces the same output and
check-chainlint self-tests pass.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>

Author: mmontalbo