[stdlib] Complete the string literals signature to match the String one

[msaelices]

To match the existing methods in both Mojo and Python strings.

This could help bit a little with the transition of Python programmers playing with the REPL and small mojo examples that handle strings, finding it more familiar with Python

E.g. it's annoying for them to start playing with Mojo with a file like this one:

fn main():
  s = "Hello"
  for c in s:
      print(c)

And encounter an error like this one:

❯ mojo t.mojo 
/tmp/t.mojo:3:12: error: 'StringLiteral' does not implement the '__iter__' method
  for c in s:
           ^
mojo: error: failed to parse the provided Mojo source module

New methods and features implemented in String literals

• Implemented .strip(), .lstrip(), and .rstrip()
• Implement .isdigit()
• Implement .islower() and .isupper()
• Implement .__iter__()
• Implement indexing
• Implement .startswith() and .endswith()
• Implement .rjust(), .ljust() and .center()
• Implement .count()
• Implement .split() and .splitlines()

Example

> mojo
1> x = "123".isdigit() 
(Bool) x = True
2> y = "  hello  ".strip() 
(String) y = "hello"
2>  z = "  hello  ".lstrip()
(String) z = "hello  "
3> t = "  hello  ".rstrip()
(String) t = "  hello"
4> b1 = "HELLO".isupper()
(Bool) b1 = True
5> b2 = "HELLO".islower()
(Bool) b2 = False
6> s = "Hello"
(StringLiteral) s = "Hello"                                                                      │
7> h = s[0]
(String) x = "H"
8> b3 = s.startswith("He")
(Bool) b3 = True
9> sl = s.ljust(10)
10> c = s.count("l")
(Int) c = 2
11> first = "Hello world".split()[0]
(String) first = "Hello"
12> for c in "HELLO":
12.   print(c)
13.
H
E
L
L
O  

[JoeLoser]

!sync

[JoeLoser]

!sync

[modularbot]

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