txtar: add fs.FS support

[earthboundkid]

From implementation copied with permission from @josharian
https://github.com/josharian/txtarfs/blob/main/txtarfs_test.go
golang/go#44158 (comment)

Fixes #44158

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Message from Ian Lance Taylor:

Patch Set 1:

(7 comments)

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Message from Carl Johnson:

Patch Set 2:

(7 comments)

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Message from Josh Bleecher Snyder:

Patch Set 3:

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Message from Carl Johnson:

Patch Set 3:

(4 comments)

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Message from Carl Johnson:

Patch Set 4:

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Message from Carl Johnson:

Patch Set 5:

(6 comments)

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Message from Carl Johnson:

Patch Set 5:

(1 comment)

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