Add wasm+npm package

This lets us compile the lib into wasm and adds a npm wrapper package for it

I had to rearrange the code slightly to split out any methods using wasm incompatible types. Not sure if there's a better way to do this

Author: mjbvz

.gitignore

@@ -3,3 +3,4 @@ Cargo.lock
 /crates/*/target/
 /crates/*/Cargo.lock
 .vscode/
+.DS_Store

crates/string-offsets/.gitignore

@@ -0,0 +1,3 @@
+node_modules
+pkg
+

crates/string-offsets/Cargo.toml

@@ -9,6 +9,12 @@ license = "MIT"
 keywords = ["unicode", "positions", "utf16", "characters", "lines"]
 categories = ["algorithms", "data-structures", "text-processing", "development-tools::ffi"]
 
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+wasm-bindgen = "0.2"
+
 [dev-dependencies]
 rand = "0.8"
-rand_chacha = "0.3"
+rand_chacha = "0.3"