Only strip the trailing source location from messages

[dpc00]

Fixes #55.

_filter_message() replaced every on and in in the message, so ordinary PHP messages were garbled, e.g. Cannot use isset() on the result of an expression became Cannot use isset()the result of an expression, and 'break' not in the 'loop' or 'switch' context became 'break' notthe 'loop' or 'switch' context.

As you said in the issue, only the trailing in Standard input code / in - (what PHP appends and regex lets through in message) should be removed. This replaces the four replace() calls with one regex anchored at the end of the message:

TRAILING_SOURCE_RE = re.compile(r'\s+in\s+(?:Standard input code|-)\s*$')

I added a test, test_issue_55_message_filter_only_strips_trailing_source, with three messages from php -l on PHP 8.4 (the isset() one, the break one, and Cannot redeclare f() (previously declared in Standard input code:2), where the in Standard input code:2 inside the parentheses now stays since it is not trailing). I ran the test module with UnitTesting in Sublime Text 4215: the new test fails before the change (with the garbled text above) and all four tests pass after it, including the three existing ones.

One behaviour change to be aware of: a standalone on that is not at the end is no longer removed. regex already consumes the on line N part, so I could not find a case where the old replacement was needed, but if you know of one, tell me and I'll add a test for it.

[kaste]

Looks legit.