[Feature] Provide more Python APIs to build grammars.

[Seven-Streams]

At present, we have several methods to build grammars: from an EBNF grammar, from a json schema, or from a regex e.t.c. This PR offers more functions to build grammar. Typically, here are:

• empty(), return an empty grammar.
• string(str), return a grammar which match the specific string.
• character_class(bool, List[char]), return a grammar which matches(or rejects) a chracterclass.
• star(grammar), return a grammar matches grammar*.
• plus(grammar), return a grammar matches grammar+.
• optional(grammar), return a grammar matches grammar?.

For example, if we want to create a grammar to match a signed integer, we can use:

import xgrammar as xgr
grammar1 = xgr.Grammar.union(xgr.Grammar.string("-"), xgr.Grammar.string("+"))
grammar2 = xgr.Grammar.character_class("[1-9]")
grammar3 = xgr.Grammar.star(xgr.Grammar.character_class("[0-9]"))
grammar1_optional = xgr.Grammar.optional(grammar1)
grammar = xgr.Grammar.concat(grammar1_optional, grammar2, grammar3)

Then, we can use grammar to match integers.

Signed-off-by: Yuchuan [email protected]