Add nodes for basic string operations

[jtydhr88]

String nodes, requested in RFC https://github.com/Kosinkadink/rfcs/blob/main/rfcs/0000-corenodes.md#string-nodes
Designed nodes:

1. Concatenate, and input is:

• string_a,
• string_b
• separator
  

2. Substring, and input is:

• string
• start
• end
  

3. Length, and input is

• string
  

4. Case Converter, and input is

• string
• mode, with options are ["UPPERCASE", "lowercase", "Capitalize", "Title Case"]
  

5. Trim, and input is

• string
• mode, with options are ["BOTH", "LEFT", "RIGHT"]
  

6. Replace, and input is

• string
• find
• replace
  

7. Contains, and input is

• string
• substring
• case_sensitive
  

8. Compare, and input is

• string_a
• string_b
• mode, with options are ["StartsWith", "EndsWith", "Equal"]
• case_sensitive
  

9. Regex Match, and input is

• string
• regex_pattern
• case_insensitive
• multiline
• dotall
  

10. Regex Extract, and input is

• string
• regex_pattern
• mode, with options are ["First Match", "All Matches", "First Group", "All Groups"]
• case_insensitive
• multiline
• dotall
• group_index