Add `string` predicates `startsWith`, `endsWith` and `contains`

What do you think about adding predicates to the CodeQL type `string` for determining whether a string has a given prefix or suffix or contains a substring? For example `startsWith(string)`, `endsWith(string)` and `contains(string)`.
Currently the workarounds are using `indexOf(...) = 0` or `matches(...%)` (which seems to be faster than `indexOf`, see https://github.com/github/codeql/issues/6479#issuecomment-900081363). However, these predicates do not convey the intention as clearly, might not be that performant and for `matches` one must take care not to accidentally use `%`or `_` where the intention was to match them literally.

In the github/codeql repository (at https://github.com/github/codeql/commit/39533317ffbeb6224d049de22fc182d8eeea4b61) there are at least:
- 196 cases where `startsWith` could be used
(I searched for the regex `matches\("[^%_]*%"\)` in CodeQL source files)
- 72 cases where `endsWith` could be used
(I searched for the regex `matches\("%[^%_]*"\)` in CodeQL source files)

该 issue 提议向 CodeQL 的 string 类型添加 startsWith、endsWith 和 contains 谓词，但没有说明源文件、测试或实现入口。首先定位 string 类型的实现及其测试；当这些谓词可用，并且覆盖前缀、后缀和子字符串检查，且不依赖 indexOf 或 matches 变通方案时，即表示完成。

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