fmt: add Join function for formatting and joining slice elements

[vikashr45]

This change adds a generic Join function to the fmt package that
formats each element of a slice using the default format (%v) and
joins them with a separator string. This complements the existing
fmt formatting functions and provides a convenient way to format
and join slice elements of any type.

The function signature is:
func Join[T any](elems []T, sep string) string

Key features:

• Works with slices of any type
• Uses fmt's default formatting for each element
• Returns empty string for nil or empty slices
• Handles single element slices efficiently

This is similar to strings.Join but works with any type and uses
fmt formatting, making it useful for formatting slices of integers,
floats, structs, or any other type.

The implementation includes comprehensive test coverage for edge
cases including empty/nil slices, various types, special values,
and different separator formats.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Vikash Kumar:

Patch Set 1:

(1 comment)

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Message from Sean Liao:

Patch Set 1: Hold+1

(1 comment)

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[gopherbot]

Message from Vikash Kumar:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR is being closed because golang.org/cl/727840 has been abandoned.

see the issue, but it seems we decided against having fmt handle slices