Substring Match Algorithm (#15511)

<!--
if this PR closes one or more issues, you can automatically link the PR
with
them by using one of the [*linking
keywords*](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword),
e.g.
- this PR should close #xxxx
- fixes #xxxx

you can also mention related issues, PRs or discussions!
-->

# Description
<!--
Thank you for improving Nushell. Please, check our [contributing
guide](../CONTRIBUTING.md) and talk to the core team before making major
changes.

Description of your pull request goes here. **Provide examples and/or
screenshots** if your changes affect the user experience.
-->
This PR should close #15474 .

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
When users set the match algorithm to 'substring' by modifying
`$env.config` to `$env.config.completions.algorithm = "substring"``),
completions are done based on substring matches.
This was previously possible by setting `positional` to be false in
custom completers, but doing so now logs a warning as this feature is
set to be deprecated and replaced by the new way of setting the matching
algorithm to substring based.

crates/nu-protocol/src/config/completions.rs

@@ -6,6 +6,7 @@ use crate::engine::Closure;
 pub enum CompletionAlgorithm {
     #[default]
     Prefix,
+    Substring,
     Fuzzy,
 }
 
@@ -15,8 +16,9 @@ impl FromStr for CompletionAlgorithm {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s.to_ascii_lowercase().as_str() {
             "prefix" => Ok(Self::Prefix),
+            "substring" => Ok(Self::Substring),
             "fuzzy" => Ok(Self::Fuzzy),
-            _ => Err("'prefix' or 'fuzzy'"),
+            _ => Err("'prefix' or 'fuzzy' or 'substring'"),
         }
     }
 }