Track column numbers (colno) in AST nodes

[daniel7an]

Add column number (colno) tracking to AST nodes, complementing the existing lineno attribute. This enables more precise source location reporting for static analysis tools.

Closes #2133

Changes:

• Add colno field to Token namedtuple (default 0, backward compatible)
• Track column position in tokeniter relative to line start
• Add colno to Node attributes alongside lineno and environment
• Add Node.set_colno() method mirroring set_lineno()
• Propagate colno from tokens to nodes throughout the parser
• Update ext.py token unpacking for compatibility

Example:

from jinja2 import Environment, nodes
env = Environment()
ast = env.parse('hello {{ foo }} world {{ bar + baz }}')
for name in ast.find_all(nodes.Name):
    print(f'{name.name}: line={name.lineno}, col={name.colno}')
# foo: line=1, col=10
# bar: line=1, col=26  
# baz: line=1, col=32

Backward compatible: Token.colno defaults to 0, Node.colno defaults to None. All 911 existing tests pass. Added 12 new tests.