feat: implement the string functions in the WDL standard library.

[peterhuene]

This commit implements the following functions in the WDL standard library:

• find
• matches
• sub

Additionally, the spans of the arguments are now passed into the functions so that any returned diagnostics may point at a particular argument.

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[peterhuene]

We should have a lint that checks to see if the pattern argument to each of these functions (find, matches, and sub) is a string literal, and if so, do a static check to ensure it's a valid regular expression.

Edit: opened #253 to track.