strings: add strings helpers for prefixes and suffixes

[testwill]

This PR relates to proposal: #66191

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from guoguangwu guo:

Patch Set 1:

(1 comment)

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Message from qiulaidongfeng:

Patch Set 2:

(1 comment)

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Message from qiulaidongfeng:

Patch Set 2:

(1 comment)

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Message from guoguangwu guo:

Patch Set 2:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 2: Hold+1

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[darth-raijin]

Use another word instead of indicates

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