Separate colors for different diagnostics types (#2437)

* feat(theme): add separate diagnostic colors

This commit adds separate diagnostic highlight colors for the different
types of LSP severities. If the severity type doesn't exist or is
unknown, we use some fallback coloring which was in use before this
commit.

Some initial color options were also added in the theme.toml

Resolves issue #2157

* feat(theme): add docs for new diagnostic options

* feat(theme): adjust defaults & reduce redundancy

- the different colors for different diagnostic severities are now
  disabled in the default theme, instead diagnostics are just generally
  underlined (as prior to the changes of this feature)
- the theme querying is now done once instead of every iteration in the
  loop of processing every diagnostic message

0 files changed, 0 insertions, 0 deletions