[ExpressionLanguage] Add more configurability to the parsing/linting methods

[fabpot]

Q	A
Branch?	7.1
Bug fix?	no
New feature?	yes
Deprecations?	yes
Issues	Fixes #50144
License	MIT

When linting/parsing an expression, two kinds of errors can be thrown: a syntax error (for which the parser cannot recover) or an unknown variable/function error (which does not block the parser).

When linting an expression, currently you can ignore unknown variables by passing null instead of the valid variable names to lint() or parse(), but you cannot ignore unknown functions. For some use cases, it might be needed to just validate the syntax.

This PR allows developers to ignore functions and/or variables the "proper way" with flags as a new argument to these methods:

$parser->lint($expr, flags: Parser::IGNORE_UNKNOWN_FUNCTIONS | Parser::IGNORE_UNKNOWN_VARIABLES);
$parser->parse($expr, flags: Parser::IGNORE_UNKNOWN_FUNCTIONS | Parser::IGNORE_UNKNOWN_VARIABLES);

Passing null to the $names argument is deprecated:

Before:

$parser->lint($expr, null);

After:

$parser->lint($expr, flags: Parser::IGNORE_UNKNOWN_VARIABLES);