docs: document String class methods and add startsWith, endsWith, indexOf natives

test/string/endsWith.wren

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+IO.print("abcd".endsWith("cd")) // expect: true
+IO.print("abcd".endsWith("abcde")) // expect: false
+IO.print("abcd".endsWith("abcd")) // expect: true
+IO.print("abcd".endsWith("f")) // expect: false
+IO.print("abcd".endsWith("")) // expect: true

test/string/endsWith_invalid_arg.wren

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+IO.print("abcd".endsWith(null)) // expect runtime error: Argument must be a string.

test/string/indexOf.wren

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+IO.print("abcd".indexOf("cd")) // expect: 2
+IO.print("abcd".indexOf("a")) // expect: 0
+IO.print("abcd".indexOf("abcde")) // expect: -1
+IO.print("abab".indexOf("ab")) // expect: 0

test/string/indexOf_invalid_arg.wren

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+IO.print("abcd".indexOf(null)) // expect runtime error: Argument must be a string.

test/string/startsWith.wren

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+IO.print("abcd".startsWith("cd")) // expect: false
+IO.print("abcd".startsWith("a")) // expect: true
+IO.print("abcd".startsWith("abcd")) // expect: true
+IO.print("abcd".startsWith("abcde")) // expect: false
+IO.print("abcd".startsWith("")) // expect: true

test/string/startsWith_invalid_arg.wren

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+IO.print("abcd".startsWith(null)) // expect runtime error: Argument must be a string.