Fix select cell path renaming behavior (#13361)

# Description

Fixes #13359

In an attempt to generate names for flat columns resulting from a nested
accesses #3016 generated new column names on nested selection, out of
convenience, that composed the cell path as a string (including `.`) and
then simply replaced all `.` with `_`. As we permit `.` in column names
as long as you quote this surprisingly alters `select`ed columns.


# User-Facing Changes
New columns generated by selection with nested cell paths will for now
be named with a string containing the keys separated by `.` instead of
`_`. We may want to reconsider the semantics for nested access.

# Tests + Formatting
- Alter test to breaking change on nested `select`

crates/nu-command/tests/commands/select.rs

@@ -50,7 +50,7 @@ fn complex_nested_columns() {
         r#"
                 {sample}
                 | select nu."0xATYKARNU" nu.committers.name nu.releases.version
-                | get nu_releases_version
+                | get "nu.releases.version"
                 | where $it > "0.8"
                 | get 0
             "#