Support both prepend and append when aggregating TOMLs

[hinerm]

jvm.root-paths determines the location Jaunch looks for Java versions.

Currently, this configuration is also specified in fiji.toml which aggregates jvm.toml. This causes the fiji.toml values to be prepended and thus completely take precedence over the jvm.toml versions - even though jvm.toml includes e.g. --java-home which should be an absolute override as a user flag.

@ctrueden suggested some alternatives:

1. add a @{app-specific-root-paths} reference... somewhere... in jvm.toml which is then an extension point that can be leveraged in fiji.toml
2. Just directly move the fiji.toml entry to jvm.toml
3. Add a flag, i.e. $ (! is already taken for inverting hints), that when present in a list item causes the value to be appended instead of prepended