Add vector.to_string and vector.from_string (#10323)

Writing vectors as strings is very common and should belong to `vector.*`. `minetest.pos_to_string` is also too long to write, implies that one should only use it for positions and leaves no spaces after the commas.

doc/lua_api.txt

@@ -3149,6 +3149,16 @@ For the following functions, `v`, `v1`, `v2` are vectors,
     * Returns a vector.
     * A copy of `a` if `a` is a vector.
     * `{x = a, y = b, z = c}`, if all of `a`, `b`, `c` are defined numbers.
+* `vector.from_string(s[, init])`:
+    * Returns `v, np`, where `v` is a vector read from the given string `s` and
+      `np` is the next position in the string after the vector.
+    * Returns `nil` on failure.
+    * `s`: Has to begin with a substring of the form `"(x, y, z)"`. Additional
+           spaces, leaving away commas and adding an additional comma to the end
+           is allowed.
+    * `init`: If given starts looking for the vector at this string index.
+* `vector.to_string(v)`:
+    * Returns a string of the form `"(x, y, z)"`.
 * `vector.direction(p1, p2)`:
     * Returns a vector of length 1 with direction `p1` to `p2`.
     * If `p1` and `p2` are identical, returns `{x = 0, y = 0, z = 0}`.