feature: add the `lsp-java-bytecode-viewer` function.

[sakurawald]

The jdtls server itself contains cfr, veinflower and fernflower de-compilers.
It may be useful to add a new command to view the byte-code of current buffer.
The byte-code level viewer is useful for these cases:

1. Debug the java source in the low-level.
2. Write java codes for a java-agent, which requires to operate on the byte-code level.

This lsp-java-bytecode-viewer function can be done via calling the decompiler provided by jdtls.