feat: improve toml formatting

[phm07]

This PR changes the TOML library from pelletier/go-toml/v2 to BurntSushi/toml. This improves the formatting of TOML files by using indents and also uses time.Duration's string values instead of writing nanoseconds into the config file.

Config format example

Before:

active_context = 'test_context'

[preferences]
debug = true
poll_interval = 1234000000

[[contexts]]
name = 'test_context'
token = 'super secret token'

[contexts.preferences]
array_option = ['1', '2', '3']
endpoint = 'https://test-endpoint.com'
quiet = true

[contexts.preferences.nested]
array_option = ['1', '2', '3']

[[contexts]]
name = 'other_context'
token = 'another super secret token'

[contexts.preferences]
poll_interval = 1234000000

After:

active_context = "test_context"

[preferences]
  debug = true
  poll_interval = "1.234s"

[[contexts]]
  name = "test_context"
  token = "super secret token"
  [contexts.preferences]
    array_option = ["1", "2", "3"]
    endpoint = "https://test-endpoint.com"
    quiet = true
    [contexts.preferences.nested]
      array_option = ["1", "2", "3"]

[[contexts]]
  name = "other_context"
  token = "another super secret token"
  [contexts.preferences]
    poll_interval = "1.234s"

Contexts are now grouped together instead of there being exactly one newline between every section, regardless of being in another context or not. Also, nested sections are now indented, which helps with understanding the structure.

Binary size

Before:

➜  cli git:(21e342b) GOARCH=amd64 GOOS=linux go build -o hcloud cmd/hcloud/main.go
➜  cli git:(21e342b) wc -c hcloud                                                 
 20606086 hcloud

After:

➜  cli git:(d336765) GOARCH=amd64 GOOS=linux go build -o hcloud cmd/hcloud/main.go
➜  cli git:(d336765) wc -c hcloud 
 20890490 hcloud

The binary size increases by ~1.4%, which is negligible.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.32%. Comparing base (d1c6678) to head (aec51fb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #758   +/-   ##
=======================================
  Coverage   59.32%   59.32%           
=======================================
  Files         203      203           
  Lines        7460     7460           
=======================================
  Hits         4426     4426           
  Misses       2400     2400           
  Partials      634      634           

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