t: extract chainlint's parser into shared module

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.  A subsequent commit adds
greplint.pl, which needs the same tokenizer to correctly identify
command boundaries.

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>