Substring Match Algorithm (#15511)

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# Description
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This PR should close #15474 .

# User-Facing Changes
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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-utils/src/default_files/doc_config.nu

@@ -115,12 +115,13 @@ $env.config.cursor_shape.vi_normal = "underscore"  # Cursor shape in normal vi m
 # $env.config.completions.*
 # Apply to the Nushell completion system
 
-# algorithm (string): Either "prefix" or "fuzzy"
+# algorithm (string): "prefix", "substring" or "fuzzy"
 $env.config.completions.algorithm = "prefix"
 
 # sort (string): One of "smart" or "alphabetical"
 # In "smart" mode sort order is based on the "algorithm" setting.
 # When using the "prefix" algorithm, results are alphabetically sorted.
+# When using the "substring" algorithm, results are alphabetically sorted.
 # When using the "fuzzy" algorithm, results are sorted based on their fuzzy score.
 $env.config.completions.sort = "smart"