Do not allow mixed (spaces & tab) indent

[Thirumalai-Shaktivel]

Example:

if sys.version_info[1] > 5:
	from typing import TextIO
else:
	from typing import TextIO

Error:

syntax error: Token 'else' is unexpected here
  --> examples/expr2.py:15:1
   |
15 | else:
   | ^^^^ 

[Thirumalai-Shaktivel]

The problem is actually with the tab here. Instead of the 4 spaces, a tab is used. So, we get the above error.

[Thirumalai-Shaktivel]

The actual issue is fixed. But, the following syntax should not be allowed

def test():
	if x > 0:
  pass
 else:
		x()

TODO:
So, we have to differentiate between the space indent and tab indent

[Thirumalai-Shaktivel]

Remaining issues to fix:
Example 1

def test():
	if x > 0:
	  pass
	else:
		x()

Example 2

def test():
	if x > 0:
		pass
	else:
	pass

In Example 1 the indent has a mix of \t and space: the problem here is that we generate the indent as soon as we see the colon followed by a newline but we don't check the indent length with the previous length and also mix of the tabs and spaces.
In Example 2 the last dedent of the scope is automatically generated, we should fix that also.