Reduce duplication in history path construction (#13475)

# Description
Currently there is a bit of chaos regarding construction of history file
paths. Various pieces of code across a number of crates reimplement the
same/similar logic:
- There is `get_history_path`, but it requires a directory parameter (it
really just joins it with a file name).
- Some places use a const for the directory parameter, others use a
string literal - in all cases the value seems to be `"nushell"`.
- Some places assume the `"nushell"` value, other plumb it down from
close to the top of the call stack.
- Some places use a constant for history file names while others assume
it.

This PR tries to make it so that the history/config path format is
defined in a single places and so dependencies on it are easier to
follow:
- It removes `get_history_path` and adds a `file_path` method to
`HistoryConfig` instead (an extra motivation being, this is a convenient
place that can be used from all creates that need a history file path)
- Adds a `nu_config_dir` function that returns the nushell configuration
directory.
- Updates existing code to rely on the above, effectively removing
duplicate uses of `"nushell"` and `NUSHELL_FOLDER` and assumptions about
file names associated with different history formats

# User-Facing Changes
None

tests/repl/test_config_path.rs

@@ -140,8 +140,8 @@ fn test_config_path_helper(
 #[test]
 fn test_default_config_path() {
     Playground::setup("default_config_path", |_, playground| {
-        let config_dir = nu_path::config_dir().expect("Could not get config directory");
-        test_config_path_helper(playground, config_dir.join("nushell"));
+        let config_dir = nu_path::nu_config_dir().expect("Could not get config directory");
+        test_config_path_helper(playground, config_dir);
     });
 }