Fix linter for encoding/wkt

[twpayne]

This is a minor follow-up for @andyyang890's excellent #197.

The changes are:

• Install the goyacc tool.
• Disable the generation of line numbers in the generated Go code, as these confuse golangci-lint.
• Replace the generate.sh script with //go:generate lines.
• Rename the generated file to match the format used elsewhere in the module.
• Format all files with gofumport.
• Replace a series of if/else ifs with a switch.