limit expression nesting depth to avoid bare RecursionError - #377

_expression() recurses once per nesting level of the parsed
expression with no bound. A deeply nested expression string
(e.g. 50,000 levels of '[') drives the interpreter's call stack to
its limit and raises a bare RecursionError, not
jmespath.exceptions.ParseError or any other member of this
library's own exception hierarchy - so a caller following the
documented error-handling pattern (catching only JMESPathError
around compile()/search()) does not catch it.

Add _MAX_EXPRESSION_DEPTH (100, well above any realistic
hand-written or generated expression) to Parser, track the current
depth across the single shared recursive entry point (_expression),
and raise a regular ParseError past that depth instead of recursing
further.

Adds 4 regression tests: a too-deep expression raises ParseError and
never leaks a RecursionError, a reasonably nested expression still
parses, and depth correctly resets between separate parse() calls on
the same (possibly cache-reused) Parser instance. Full existing test
suite (995 tests, including the upstream JMESPath compliance suite)
passes unchanged.