Optimize language service build (#520)

`tsdx` uses `.browserslistrc` to determine how to transpile the source
code. By default it supports very old environments.

By specifying relatively modern targets, the bundle size has been
reduced with ~40%. This also significantly increases performance and
readability of the output, because babel’s regenerator runtime isn’t
used anymore.

The output target can’t be set to something too new, because `tsdx` uses
an older version of terser which isn’t compatible with all new syntax
(e.g. optional chaining).

packages/tailwindcss-language-service/.browserslistrc

@@ -0,0 +1,4 @@
+last 2 Chrome versions
+last 2 Firefox versions
+last 2 Safari versions
+node 12