Add string.startsWith and string.endsWith

[grumpyoldman-io]

This adds a startsWith and endsWith helper on the string like discussed here #604

small aside, the playground.ts is missing, which is mentioned in the contrib

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[grumpyoldman-io]

CC @alii

[alii]

ooh awesome :D thank you!

[grumpyoldman-io]

@colinhacks anything else I can do for this PR, I cant assign you as a reviewer and the contribution file didnt mention anything around PR creation guidelines.

[AustinShelby]

Transforming the parameter users give to the startsWith function into a regex doesn't work in certain edge cases.

Consider the following scenario;

const x = z.string().startsWith("[a-g]");
x.parse("[a-g]");

By first look this should work as the string [a-g] starts with [a-g].

The problem is that when you transform [a-g] into a regex using your implementation it checks if the string starts with any character from a to g, which it doesn't as it starts with the character [.

I suggest using the native String functions startsWith and endsWith.

[Svish]

Rather than regular expressions, depending on what browser support this library aims for, these rules should probably just use the built-in startsWith and endsWith string functions, or alternatively the older indexOf of lastIndexOf functions?