Allow language context to be externally provided

[nikku]

Proposed Changes

When using the FeelEditor provided by properties panel folks can now inject a languageContext, to provide custom builtins, dialect and parserDialect:

<FeelLanguageContext.Provider value={ { parserDialect: 'camunda' } }>
  <FeelTextfieldComponent .../>
</FeelLanguageContext.Provider>

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

Draft, because test cases are missing.