Improvements in diagnostic handler

[nenikitov]

More customization in diagnostic handler

Exposed more configuration parameters (signs and min_severity). Both can be configured with setup:

require('satellite').setup {
    diagnostic = {
      signs = {'-', '=', '≡'},
      min_severity = vim.diagnostic.severity.HINT,
    },
}

Added min_severity option

This option specifies the least important diagnostic that should be shown on the scroll bar. By default it is set to HINT, which means all diagnostics are shown. But, if set to INFO, it will show everything including INFO, but not HINT. Example:

• Set to HINT
• Set to INFO

Better highlights

Before, the diagnostic sign was highlighted with the severity of the last diagnostic in that part of the file. Which means that, sometimes, the lines with multiple diagnostics wouldn't be highlighted properly:

• Before (the line has both warning and error, warning is shown on the scroll bar)
• After (error is shown on the scroll bar)