parser: +feat Choose the post-processing of the regex conversion with Re2Options

[Adda0]

create_nfa() tied the conversion and its post-processing together
through positional booleans: it always trimmed the result, removed
epsilon transitions unless use_epsilon was set, and reduced it with
use_reduce, so a caller could not get a raw conversion out of the
parser, and builder::create_from_regex() and the Python from_regex()
exposed only the reduction.

The new Re2Options struct (keep_epsilon, epsilon, encoding, trim,
reduce) is taken by new overloads of create_nfa() and
builder::create_from_regex(). Its defaults reproduce the previous
behaviour, and the existing overloads forward to it with a trimmed
result, so no current caller changes. The Python from_regex() gains
the trim and keep_epsilon keyword arguments, with defaults matching
the previous behaviour.

Part of #793.

struct Re2Options {
	bool keep_epsilon{false};
	Symbol epsilon{306};
	Encoding encoding{Encoding::Latin1};
	bool trim{true};
	bool reduce{true};
};
nfa::Nfa create_nfa(const std::string& pattern, const Re2Options& options);
Nfa builder::create_from_regex(const std::string& regex, const parser::Re2Options& options);

parser.from_regex(regex, encoding='utf-8', reduce=True, trim=True, keep_epsilon=False)

The existing create_nfa(pattern, use_epsilon, epsilon_value, use_reduce, encoding) overloads and create_from_regex(regex, bool reduce = true) are unchanged and keep their behaviour.

Verification

• C++ test mata::parser::create_nfa() post-processing options: the defaults give the same automaton as the old overloads, both directly and through create_from_regex(); keep_epsilon keeps epsilon transitions labelled with the requested symbol (500), and none labelled 306; trim = false keeps the useless state of a[^\x00-\xff]; encoding reaches the parser (UTF-8 \x{100} is c4 80, and Latin1 rejects it).
• Python test test_post_processing_options covers keep_epsilon and trim.
• This PR changes only the API, so there is no performance claim. The conversion pipeline itself is the same.

Stack

Second of three PRs for #793. Based on #948 (opcode handling), so the base of this PR is that branch. GitHub retargets it to devel once #948 is merged.

Overlap:

• open PR nfa: +feat Make State 32-bit by default and check state overflow #939 changes casts in the start_state lines of convert_pro_to_nfa, which the third PR of this stack removes;
• parser: +fix Malformed .mata input only validated by asserts, crashes Release builds #877 edits other declarations in parser.pxd;
• bindings/python: +fix Reject .mata files holding more than one automaton #895 edits from_mata in parser.pyx, next to from_regex;
• nfa,nft: +feat Add a checked delta builder and a batched transition import #883 and parser: +fix Malformed .mata input only validated by asserts, crashes Release builds #877 touch src/nfa/builder.cc in other functions.