Fixes #5379.

This renames the parser to add a new member variable describing the lexer mode: Default or inside character class. This allows the lexer to correctly process a backslash when parsing a character class/bracket expression.

I also tried to do this without renaming the parser, but this would mean we would have to pass the lexer mode (in or outside a character class) as an argument to all the functions processing escapes in any way, which is a bit of a pain. By renaming the parser, we need the least changes to the logic itself.

Since the parser is renamed, this PR is also doing a number of minor cleanups to the parser and builder (which is also renamed to do these cleanups).

The PR is split into several commits to simplify reviewing:

• Rename _Parser to _Parser2.
• Since we have renamed the parser, we can strip any version numbers from member functions.
• Clean up the parse flags, which we can do now because there is no longer any chance of mix-and-matching the parser constructor and the parser member function _Compile. Specifically:
  • _L_brk_bal is assigned its own bit; previously it was

    STL/stl/inc/regex

    Line 1879 in cbd091e

    _L_brk_bal = 0x20000000, // ']' special only after '[' (ERE, BRE); TRANSITION, ABI: same value as _L_brk_rstr

  • The _L_grp_esc flag is added to the awk flags so that the workaround in _ClassAtom can be removed.
  • I also extended the _Lang_flags enum and the _L_flags member variable to unsigned long long so that we can add more flags more easily in the future. (This already adds _L_dsh_rstr to signify that the dash - cannot appear as the starting point of a character range in BREs and EREs, but doesn't perform the parser changes to support it yet.)
• Remove the unused member _Begin from the parser.
• Slightly reorder the parser member variables to reduce padding a bit. (_Char is usually a char or wchar_t, so it [plus the single-byte _Mode member variable added in the last commit] can usually fit into the four bytes the compiler must add after _Mchar.
• Rename _Builder to _Builder2.
• Strip version numbers from member functions of the builder.
• Remove obsolete members _Bmax and _Tmax from the builder.
• Actually fix <regex>: Backslashes in character classes are sometimes not matched in basic regular expressions #5379 essentially by making _Is_esc() always return false when not in default (read: outside-bracketed-character-class) mode. Note that it matters how we change the lexer mode in _Parser2::_Alternative(): _Next() and _Expect() process the first token inside or outside the square brackets, so we must change the mode before calling these functions. The tests check that we didn't get this wrong.