Consider implementing string functions

[springcomp]

The string functions are currently missing from this crate.
Consider implementing string functions.

From the JEP 14 – String Functions specification, identify all the specified functions. For each one:

• Create a corresponding source file for the function in src/jmespath/functions/.
• Make sure to add some unit-tests, taking each of the function examples from the spec under consideration.
• Export that function from the module in src/jmespath/functions/mod.rs.
• Create pull request for the function under consideration (one PR per function).

As a sanity check, please run the compliance tests.
First make sure compliance tests are included from the Git submodule using the following command:

• make compliance

Then, enable the functions_strings.json compliance test suite by momentarily removing it from the list of currently excluded files.

Re-run make compliance to check that all string functions run successfully.

Please, make sure to not commit the temporary enabled functions_strings.json compliance test suite for now.