added import_style parameter to generate import statements for browser that support es6

[safanaj]

Changes

• added a import_style parameter with the only meaningful value es6, it affect only service=grpc-web generation code. The purpose of this is generate import/export code that doesn't need browserify,webpack or similar bundler to work in browser that supports es6.
• in the test/helpers/fakeGrpcTransport.ts I just changed new Buffer to Buffer.from due this depracation warning.

I need this to use protoc-gen-ts to generate service js file usable in sveltejs + vitejs (rollup bundler).

Passing the import_style param like --ts_out=service=grpc-web,import_style=es6 the require(...) statements are replaced by import * as ... from ... statements, and exports. .... = ... by export {...} statements.

Verification

I checked it with a tiny sveltjs+vitejs application.

[improbable-prow-robot]

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[safanaj]

/assign @marcuslongmuir